For blog postings specifically, I love the fact that I have the British English dictionary add-on installed. It spell-checks everything that I type into text boxes (such as the one I'm typing into now).
Other Extensions (which is an Add-on sub-type) that I can't live without are:
- Google Toolbar, which just makes life so much easier
- FlashGot, for letting me choose which program I want to use for downloading something
- DownThemAll!, for letting me download multiple links/elements in a web page
- PDF Download, for letting me choose between displaying or downloading PDF files when I click on a .pdf link
- Web Developer, for letting get into the nitty gritty of websites
- ColorZilla, for letting me identify any colour that I see in the browser window
- MeasureIt, for letting me measure (in pixels) anything that I see in the browser window
- Tabbrowser Preferences, for letting me control my Firefox tabs better
Oh, and the Long Titles extension, which lets me read long lines of alternate text (for images), is also useful for websites like xkcd.
Actually, you know what? Firefox itself is awesome :)