I have just disovered a feature that I needed sooooooooooo much @ work when having to access multiple networks with different proxies; this feature is ‘Profiles’. You can have many Firefoxes on the same OS :), each one with totally different settings, plugins, proxy, homepage, passwords, history…etc, here are the steps (for Ubuntu):

1. Close ALL open Firefox windows
2. Open a terminal and type “firefox -ProfileManager”
3. Create new profiles as you need and don’t forget to unselect the option of not prompting in order to be prompted for the profile you need each time you start firefox
4. Enjoy!