Firefox

Extensions

Nuke Anything Enhanced. Lets you remove any item from a web page. Useful for sites that insist on putting ads or other images on what are supposed to be printer-friendly pages.
LiveHTTPHeaders
Pornzilla. Some useful things there even if you don't use it for pr0n.
Paste and Go
Web Developer Extension
SpellBound - Spellchecker for Firefox and the Mozilla Suite
How'd I Get Here
foXpose. Like the Mac's Exposé feature, but for Firefox.
Greasemonkey
SessionSaver (installer here, homepage here)
DownThemAll looks neat. Have to try. I wonder if it can download embedded movies and such, rather than making me have to figure out the link for it and stick it in an HTML page so I can do a "save as". Update: nope, you can't, but if you go into "View Page Info" (right click on a page to get the menu) you can get a list of the media linked from the page and "save as".
Pearl Crescent Page Saver. Lets you save an image of a page in FF 1.5 or higher.
FireBug - truly excellent HTML/CSS/Ajax/Javascript debugger. Comes with an interactive console that lets you type in any Javascript and see the result.

Ones I have installed: How'd I Get Here, Live HTTP Headers, foXpose, SessionSaver, Nuke Anything Enhanced.