The main drawback in using the newly released FF Betas is the delayed support provided by the extension writers. Hence, many of you may be wanting to run two versions of FF simultaneously, thus enjoying the power of your favourite extension as well as taste the new innovations. Here's the way to do it:
1) Select your FF shortcuts and right-click and click properties. In the Target box, add the following options: -profilemanager -no-remote. So your full path now looks like
"XXX\firefox.exe" -profilemanager -no-remote.
Do this for all the installed versions of FF.
2) Start FF. The profile manager will start automatically. The default profile is the one which was already existing and will contain all the customizations that you have for your old FF installation (say 3.6.x). So, create a new profile for your new FF version (say FF 4b4). Choose between profiles depending your use.
This workaround has been around for quite some time now, and you will find many posts on the net. I have been using this feature since 2005, when a friend of mine, Indradeep, showed it to me. Still, thought of noting it down. I don't know why Mozilla doesn't have the Profile Manager enabled by default.