There are some must have Add-Ons that every web developer should have no matter what. Then there are a few essential Add-Ons for web masters to monitor and optimize their web pages and then there are other "icing on the cake" Add-Ons that you will love to play with.
Starting with the essential Web Developer Add-Ons, here is the list:
The name says it all. This is another awesome Add-On for Web Developers and UI Engineers to monitor the rendering of a webpage with the help of several tools lined up on the tools panel. Web Developer lets you control different aspects of a web page including the CSS modules of a page, controlling "forms" appearing on the page, minimizing page load times by optimizing images that render on the webpage and a lot more features. It supports Windows, OS X and Linux environments.
If you want to take a closer look at how your web pages are performing on Firefox (and other browsers of course) then YSlow is the definite choice. It dissects every element of a webpage, from the simplest of HTML tags to CSS style sheets, to images and displays the combined performance and rates the web page according to a defined rule set. You can define your own performance rule set through which performance of a webpage would be measured. Not only does it analyze the performance of your webpages but it also suggests possible changes to help improve their performance. YSlow uses the FireBug engine in the back end and supports Windows, Mac OS X and Linux environments.
5. CSS Viewer
It's really handy for web designers to inspect individual CSS elements of a webpage especially when designing pages or taking inspiration from a design or even when you just want to do a straight copy too. CSS Viewer integrates into Firefox and lets you inspect each CSS element when you simply hover over it. You may turn it off when you are just browsing and don't want to inspect CSS elements of the webpage.
The name says it all. If you didn't know, you can play with Firefox's advance settings but if you do not want to ruin your current settings and browsing experience then Fasterfox lets you do that in a more user friendly manner. It allows you to tweak network settings and through it's smart "pre-fetching" option, it caches all the links of the page you are visiting. Flash Ads blocker is also bundled with this Ad-On.
FireFTP is the FTP client that you can easily integrate into your Firefox and it lets you transfer files over FTP with advanced features such as SFTP, SSL 32-bit encryption, Folder comparison, Directory syncing etc. This FTP client is super light and does not need to be started separately when you want to do transfers over FTP. It matches any other advanced desktop-based FTP client in functionality and performance.
This is yet another Ad-On that is targeted for Firebug users. It lets you log your Firebug consoles, which helps in debugging the AJAX code snippets in your pages without interfering with the normal webpage rendering. It defines a separate header for communicating debugging data. You need to have Firebug and a PHP library pre-installed to make use of this Ad-On.
For web designers, grid based designs are the way to go and it's one of the most common design practices being used by websites of the like of Yahoo, Wired, The New York Times etc. Gridfox integrates into Firefox and let's you analyze the grid base of any webpage by letting you lay the grid of your choice over the page and see if the elements conform to the grid based design.
10. Pixel Perfect
It would be better to classify Pixel Perfect as Firebug Add-On rather then Firefox Add-On since it allows you to monitor pixel information of an image in real time while Firebug engine is running. Besides this, you can lay your complete designed composition over an existing HTML webpage and see the results.
Spelled Code-ech. It gives you the feel of Dreamweaver right inside Firefox by letting you view, edit and save documents even while you're browsing. You will need to have Firebug pre-installed to make use of Codetch. You can open the complete element structure of your webpages and inspect and edit elements of it and then preview them on the side panel.
For dynamic webpages, web developers often have to inspect headers of the page and usually they do it manually by inspecting the source of the returned pages. With Live HTTP Headers Firefox extension, you can view HTTP Headers as they come in while the page is loading.
13. SEO for Firefox
How could we miss SEO Add-Ons for Firefox? SEO for Firefox is among the most popular and comes packed with a whole bunch of stats tools that helps SEO experts and webmasters to keep a watch on the rankings of their pages and also to analyze and optimize the SEO performance of their web pages. It lets you monitor your Alexa and Compete traffic rankings, Google PR, Link density on your pages, Keyword analysis etc.
X-Ray let's you view the source for pages that you have opened on Firefox without doing the extra two clicks for viewing the source of the webpage. It displays the source next to the page itself. It has not been updated for Firefox 3.X so you can use it only if you have an older version of Firefox.
Aardvark is no ordinary Ad-blocker. It removes elements of the page, of your choosing, for better viewing as well as extracting the printer friendly version of a webpage. Not all webpages have Print versions and if you want to print a webpage you have to go through a cumbersome way of extracting text and images from it and then printing them. Aardvark lets you do that in an interactive manner right from Firefox.
If you want to take screenshots of webpages and save them in multiple sizes and maybe also add text on them then you can do this all right inside Firefox with FireShot extension for Firefox. Unlike other screenshot taking tools, FireShot lets you edit them as well like adding text or image connotations to the partial or full webpage screenshots.
18. IE View
Multi browser viewing capability is a must-have for any good web-design or development tool because designers need to see how the pages look like on other popular browsers. With IE View extension for Firefox, all you need to do to view pages on IE mode is simply right click on open pages and click "View in IE".
19. Opera View
Just like IE View, Opera View also makes use of the Flock engine to allow you to view the pages you have currently opened on Firefox in Opera. Obviously, you need to have Opera installed because this will use Opera's rendering engine to render the webpage.
20. Total Validator
A web design is professionally created only when it's HTML and CSS elements as a whole pass the validation rules set by W3C. With Total Validator Firefox extension, web designers can check whether their designed pages are following the validation rules. It's useful for checking validation from HTML version 2.0 to XHTML 1.1
Poster if for web developers who want to generate and send standard GET and POST requests along with all the parameters and headers required and also get responses. Developers can set complete body inside the POST request. This Firefox extension comes in handy for interacting with Web Services and other dynamic webpages.
Besides inspecting HTML and CSS elements of a webpage, designers also need to pick and choose colors of multiple pixels in a page and save them in a custom palette for later use. With ColorZilla extension for Firefox, you can view color information for any pixel on a page, measure distance between two pixels on a webpage, pick and drop pixel colors on a custom palette besides other features.
This is an awesome creative Firefox extension which lets web designers extract color schemes from any image from any webpage they have opened and save that scheme to be used in any of their designs. We all know that color aesthetics require that the images used in a webpage have to follow the color scheme of the page and this is where you can use ColorSuckr.
24. Palette Grabber
Palette grabber is yet another Firefox extension aimed for web designers to extract color information of the web pages they visit on Firefox. It let's them generate a palette that can be used in any design tool including Dream Weaver, Photoshop etc. The palette is generated based on the colors of the elements including images in the webpage.
25. Font Finder
Font Finder Firefox Add-On is for web designers and typographers who want to grab, edit and preview different fonts on an opened webpage. This plug-in gives extensive font information and allows designers to try different font styles on a webpage to see which one goes with the theme of the page.
This comes in handy for synchronizing all of your login IDs, passwords, and bookmarks across browsers and computers. This Firefox Add-On is not only for developers and designers but for anyone who wants to synchronize everything they browse across multiple machines. It also adds rankings right next to Google and other search engine results. It doesn't just sync bookmarks but will also 'optionally' sync tabs you've opened on Firefox.
Once you have all or most of the above listed extensions installed, you definitely need a way to back them all up in case your OS crashes. FEBE which is short for Firefox Environment Extension backs up all your Firefox Add-Ons and creates their .xpi files which means you can do a clean install of these Add-Ons on other machines as well.
This Firefox Add-On lets you take measurements of any element on a webpage. The measurements are taken in pixels and you can place the scale anywhere on the page. This extension definitely comes in handy for web designers creating multiple versions of their designs targeted for different screen sizes.
29. Window Resizer
Window Resizer Firefox Add-On also lets web designers to test their designs for multiple standard screen sizes and see whether the alignment is maintained across all the popular screen sizes. The screen sizes to which you can scale your pages are: 640x480, 800x600, 1024x768, 1280x800, 1280x1024, and 1600x1200. The procedure to switch to any resolution is pretty simple; a button is integrated right next to the address bar of Firefox.
Coming last on the list does not mean it's a useless Add-On. In fact Dust-Me Selectors is pretty essential for Design Usability and Optimization jobs where you have to extract unused CSS selectors on your pages so that the CSS elements that are taking idle load time are removed from the pages.