I hope this is a trivial problem and googling it around gave me no trivial solution 😦
Faced by many web-developers around the world, this context switching issue is definitely a time-sucker ! You lose the session, the cookies, the password, the settings, the plugins and a lot more than that, the time.
Coming to solution, I use portable firefox personally (if you don’t know what this, check out ProtableApps).
To run parallel firefox versions you just need to make a few changes in the configuration.
1. Goto folder \Other\Source and copy FirefoxPortable.ini to your firefox portable root, where the FireFoxPortable.exe is located.
2. Open the file in editor and change the following line:
(Basically we are allowing multiple instances of FireFox)
PS1: If you have installed both version of firefox as portable you need to make changes in both the installations. (Simply copy the same FirefoxPortable.ini to other folder.)