Switchproxy is one of my favourite firefox extensions. Unfortunately it hasn’t been updated for firefox 2. It works though – just a version check, and someone has hacked the installer.
I’ve put a local mirrored copy here.
Update 2: You can bypass firefox version extension checking, details here.
Update 3: 10th November ’06 – Blimey 7000 downloads and counting!
Update 4: I see a sanctioned release of SwitchProxy that works with FireFox 2 is out: 1.4.1