My favourite firefox extensions so far [updated]:
- NoScript Allows JavaScript and Java execution only for trusted domains and offers whitelist based pre-emptive script blocking.
- Greasemonkey Add bits of DHTML ("user scripts") to any web page to change its behavior.
- Tab Mix Plus Lets you adjust tab handling the way you need it and adds session management and crash recovery as well.
- Adblock Plus Get rid of all those annoying iframe-, banner-, remote- , flash and java ads.
- Web Developer Adds a toolbar and a menu entry with helpful stuff for developing webpages.
- Mozex Use external programs to edit textareas, handle mailto, telnet, ssh, ftp,...
- Flashblock Supress flash using a placeholder and load it when the placeholder is klicked
- SSL Blacklist Detects and warns about certificate chains that use the MD5 algorithm for RSA signatures as well as certificates with the Debian #363516 SSL bug