Firefox supports profiles: each profile is a distinct configuration under ~/.mozilla/firefox, with its own extensions, themes, bookmarks, history, and cookies. This means that badly-behaved web pages in one profile will not be able to read the data from another profile (unless they actually own your system, which is beyond the scope of this simple technique). It also means you can have as many GMail accounts as you like without having to sign out and wipe cookis every time.
To create non-default profiles, open the profile manager from the command line:
bash# firefox --no-remote -P
Create as many profiles as you like. There can be one for each online persona, one for email, one for paying bills, one for pr0n, whatever. For this discussion, we will assume the profile test was created.
For each profile, do the following:
1. Create a desktop file for the profile in ~/.local/share/applications . Make sure the .desktop filename is unique.
bash# cp /usr/share/applications/firefox.desktop ~/.local/share/applications/firefox-test.desktop
2. Add a distinct icon for the profile in ~/.local/share/icons . You can get different icons by doing a Google image search for firefox icon. There are plenty.
bash# cp ~/downloads/firefox-green.png ~/.local/share/icons/firefox-test.png
3. Edit the .desktop file to ensure that the Name and Icon are unique, and that the Exec line references the new profile:
Name=Mozilla Firefox (testing)
Exec=firefox --no-remote -P test
That's all there is to it. A new icon for Firefox (testing) will appear in your favorite window manager. Add whatever extensions you deem fit for the purpose.