One of the huge advantages of Mozilla FireFox , as compared to other browsers, it was, it still is, to possess a large amount of free extensions that allow you to customize and extend the application. As a web developer I can not - now - 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 real-time debugging of Web pages on the Internet today. Certainly exceeds in usability and functionality (as well as in Plugins and extensions available) also control tools supplied with WebKit (see Safari and Chrome) and the new Explorer 8.
Web Developer
Along with this FireBug extension allows you to activate a handy toolbar with many functions, very useful for debugging and monitoring of pages: cache control, meta, css styles, 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 recently discovered and I was really amazed at the precision and ease of use. Allows you to perform a number of checks on the syntax of the pages so as to blow out errors or markup tags used poorly or not closed. Convenient especially those who develop with WordPress or similar systems, where a page is built with various parts (header, body, footer and sidebar) difficult to examine and display as one.
Note: This extension is indicated on the repository as Mozilla NOT available for MacOS systems. However, if you go to the page developer , you find all versions instead. In fact I use it safely on my Mac!
YSlow
Manufactured by Yahoo, YSlow is an add-on FireBug and allows for extremely advanced in the pages of the report that we are developing! With lots of votes! From the analysis it is possible to obtain a series of suggestions - often very useful - to improve the stability and performance of our Web pages
CodeBurder
This extension is present both as an add-on for FireBug, both as a stand-alone extension for FireFox. Development are also versions in Adobe AIR and Adobe Dreamweaver. Code Burner provides a contextual Help - evolved - the HTML markup and CSS, indicating the cross-browser compatibility, general information, syntax, and another example:
Like you point out 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
@ Emanuele: interesting ...
nice alternative ... i try it now 
Um ... sorry but I must point out a spelling error:
"Do without some cool espensioni"
you should replace the "p" with a "t".
Hello.
@ Miki64: oops ... right ... thanks
Point out, for completeness, also the Plugin Aviary that provides all the functions of screen-capture! It runs on Mac OS and, diferrena of many, offers additional services such as online screen-capture via URL and online editor written in Adobe Flex: Very well made, allow you to quickly add more info to our screenshots!