In Firefox, I often use several accounts of the same webapp in parallel.
For instance a Twitter account for each of my projects, and several Gmail accounts for various purposes I don't want to mix.
Right now I use different browsers, and setting up each browser to my tastes takes a considerable time. I also tried running Firefox from different UNIX user accounts, but that's quite a pain to run.
Is there an add-on (or browser, or anything) that would allow me to do all of this in the same Firefox window?
I am also open to switching to a different browser, as long as it is an open source browser.
If the solution is an add-on, I prefer the add-on to be open source and if not at least free.