A large part of my job involves testing websites for various things. Most often, I use a series of Firefox add-ons and some other external tools (proxy server, for example) to perform the tasks I need to do.
Regularly, this involves clearing all cookies and personal data from my Firefox profile. I need to ensure cookies are set correctly and it's tough to do that when all of my regular browsing cookies are there too. Moreover, clearing them all when I need to perform testing requires that I then log into each site I go to regularly again. This makes me a sad panda.
I began browsing the Firefox Add-ons site to try and find an extension or other tool that would allow me to switch profiles on the fly so that I could keep my regular Firefox profile, and all the sites thereunto visited, safely tucked away and ready for my usage at my command all the while maintaining a profile that I could mercilessly abuse in the name of testing.
Of course, such a thing does not exist. However, all is not lost. The Mozillazine has an article on profile management that essentially outlines the method by which the built-in profile manager can be accessed.
As noted above, I am a whiny Firefox user. I do not want to be prompted by the profile manager each time I start Firefox. The solution to this is to have a separate Firefox shortcut with the -profilemanager flag set. I store this in with my regular application shortcuts so that it is easily accessible when I need it. Your profile selection is persistent across sessions, meaning that normal Firefox shortcut will open Firefox with the last profile you selected in the profile manager.
This allows for completely separate add-ons, themes, home pages, connection settings, security settings, and toolbar setups. I've found this incredibly useful in my work and it might service those of you who develop web applications or otherwise tool around on the Internet.