The Web Developer extension includes realtime CSS editing.
This validator puts a nice check box thing on the bottom right, and updates view source. Right Click the Yellow Triangle and select Show Text.
Aardvark lets you view the source of a specific spot on a page. Among other things. The key commands are on their website.
LinkChecker helped me tons to keep the song quotes links live. It goes through and colors the good links green and the bad links red.
IETab makes it that you never have to open IE again! If you have a site that is IE specific, you can open it in an IETab in Firefox and even specify to always open that site in IETab. Also, clicking the firefox logo in the bottom status bar switches the current page to IE. That is rocking.
Fasterfox applies those network tweaks that make firefox faster. It also included a popup block for those nasty popups that sneak in via flash.