One of the huge advantages of Mozilla FireFox , compared to other browsers, it was, it still is, to possess a large amount of free extensions that allow you to customize and extend. As a web developer I can not - now - to do without some useful extensions that help me in the construction, inspection and maintenance of Web pages
FireBug
A "must" for all Web developers is probably the most advanced editing and debugging real-time Web pages on the Internet today. Surely exceed in usability and functionality (as well as in the Plugins and extensions available) tools will also be provided with control WebKit (see Safari and Chrome) and the new Explorer 8.
Web Developer
Together with this extension FireBug allows you to enable a convenient toolbar with many features, very useful for debugging and monitoring of pages: cache control, meta, styles, css, javascript engine, cookies, form, immgaini, highlighting tools, measures, resizing the browser window, check the generated source and any frames iframe, etc ...
HTML Validator
This extension I discovered recently and I was really amazed at the precision and ease of use. Allows a series of checks on the syntax of the pages so as to jump out of errors or markup tags used improperly or not closed. Especially convenient to those developed with WordPress or similar systems, where a page is built with several parts (header, body, sidebar and footer) are difficult to examine and display as one unit.
Note: This extension is indicated on the Mozilla repository as NOT available for MacOS. However if you go on the page developer , you will find all versions instead. In fact I use it happily on my Mac!
YSlow
Manufactured by Yahoo YSlow is an add-on FireBug and allows for very advanced in the pages of the report that we are developing! With lots of votes! From the analysis it is possible to obtain a number of suggestions - often very useful - to improve stability and performance of our Web pages
CodeBurder
This extension is present both as an Add-on for FireBug, stand-alone or as an extension for FireFox. They are also developing versions in Adobe AIR and Adobe Dreamweaver. Burner Code provides a contextual Help - evolved - HTML markup and CSS, indicating compatibility between browsers, general information, syntax, example etc:
Like you point out of YSlow Page Speed , an optimizer developed by Google (initially for internal purposes and then released to the public!).
HTML Validator is very useful!
Hello,
Emanuele
@ Emmanuel: interesting ...
nice alternative ... try it once 
Um ... sorry but you point out a misspelling:
"Do without some useful espensioni"
You should replace the "p" with a "t".
Hello.
@ Miki64: oops ... corrected ... thanks
I point out, for completeness, even the plugin Aviary , which provides all the functions of screen-capture! It runs on Mac OS and, in many diferrena, offers additional online services such as screen-capture via URL and online editor written in Adobe Flex: Very well done, allowing you to quickly add more info to our screenshots!