Note: these are actually called add-ons/extensions, but old habits die hard.
Note: I've found these to be generally flaky and run with them disabled until I need them. I'm also concerned about the security risk of untrusted code running in my browser.
(May 2012) Adblock Plus 2.0.3 is fantastic. I used to resist ad-blocking because I do some web-dev and wanted my view of the web to be like a customer's view, but that hideous ad of a young woman peeling off an old woman's face pushed me over the brink. Now that I've seen the ad-free web, there's no going back.
(May 2012) Firebug 1.9.1 was fantastic, but as of Firefox 11.0, the built-in [ Tools > Web Developer > Inspect ] has been good enough for me.
(May 2012) Html Validator 0.9.5.2 is fantastic. It adds a validation pane to the existing "View Page Source" via existing firefox logic, so it's fast and easy to use. It doesn't appear in firefox's "Get Add-ons" search on OSx10.7.3 and when viewed from here states that it's "not available for your platform". This caused me to try Total Validator, which sucks. Happily, Html Validator is still manually available at skynet. Download > Mac Os X > Download > Allow > Install Now > Restart Now > View Page Source > Works Great. I guess it's just not officially supported on Firefox 11 yet.
(May 2012) Spell Checker 1.3.2 is a great time saver. I write blog posts in raw html and can't switch to google's compose view because it converts my <p> tags into <br/> tags. This addon is an easy way to spell check the result after your initial post. Note: in order to spell-check a page with frames, you must right-click in the frame then check spellings. Note: search for this in firefox addon manager via "spell", It didn't appear when I searched via "spell checker".
(May 2012) I tried MeasureIt 0.4.10, which had the best screen-shot, but after installing it, there was no way to use it. No preferences, no menu item, no toolbar, no right-click menu. I guess it just doesn't work on Mac. Next I tried Page Ruler 0.2.1, which is fairly wretched, but gets the job done. It is only accessible via an icon in the [ View > Toolbars > Add-on Bar ]. Then I got spooked about these extensions containing malware, so I started reading the reviews, and one guy pointed out that on a Mac, you can use screen shot to measure. Command-Ctrl-Shift-4 and drag to select an area. The pixel dimensions are displayed and the screen-shot is saved to the clip-board, so there's no file to delete.
(May 2012) Total Validator 7.2.1 requires an additional manual download and install of their basic tool (or pro tool for a fee). This process was launched when I first clicked the validator icon in the bottom bar. Install on OSx10.7.3: download > open with disk mounter > drag TotalValidator.app to Applications/Utilities > authenticate > double-click TotalValidator.app to launch > yes open downloaded app > exit. Then in the Total Validator Options, browse to the TotalValidator.app. Finally, use the bottom-bar icon to "Validate Page". It thinks for a long time, then nothing happens! (although I was sometimes able to get a result when running the tool manually).