My favourite firefox extensions so far:
- NoScript Allows JavaScript and Java execution only for trusted domains and offers whitelist based pre-emptive script blocking.
- Mozilla XForms Implementaion of the W3C XForms 1.0 SE recommendation.
- Greasemonkey Add bits of DHTML ("user scripts") to any web page to change its behavior.
- Fasterfox Performance and network tweaks like prefetching links, simultaneous connections, pipelining, DNS cache.
- 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.
- FoxyProxy Define multiple proxies, easy switch between them and define which one to use based on regex.
- XML Developer A toolbar helping when developing XML stuff.
- Web Developer Adds a toolbar and a menu entry with helpful stuff for developing webpages.
- User Agent Switcher Helps to get along with broken sites that check for a specific browser.
- 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
- PDF Download Chose between open and download of a PDF which is especially useful if you have Adobe Reader installed.