I am usingFirefox 2 for Development and daily work. For the Zimbra Admin Userinterface I use Firefox 1.5.0.7 Portable Edition to keep the Admin work seperate from the daily work.
You can use this trick to have Firefox 2 as main application and FF 1.5 portable for Admin User Interface work.
Run Firefox 2.0 and 1.5 side by side | Muffin Research Labs by Stuart Colville
read the instructions there and the comments carefully to save your settings and prevent from running both at the same time.
I use it now until 4.5.7 arrives.