You can use both versions but should use a different profile for each one (location where Firefox stores user data such as bookmarks).

The beta version should by default be installed in a different location than the release version, but to ensure this is the case you can choose the Custom option when installing Firefox and specify where you want it installed.

Next you need to create a new profile folder for the beta version, for details of how to do that see the Creating a new Firefox profile on Windows. Once you have created a new profile, you can create shortcuts that open the correct profile, for details see Shortcut to a specific profile.

Finally you can arrange for Firefox to synchronize the bookmarks and passwords used by the 2 versions. You can do this by using Firefox Sync. The Firefox 4 beta will have it built in, but for Firefox 3.6.10 you can install the Firefox Sync add-on. For details on using Firefox Sync see what is Firefox Sync.

IMO, you're better off using Firefox Portable 4.0 beta - it uses its' own Profile automatically.

http://portableapps.com/apps/internet/firefox_portable

http://portableapps.com/apps/internet/firefox_portable/test

Thanks a lot Edmeister! Is there any draw-back in using the portable version or any other major difference to the standard beta4? Thx

The cache is turned off, and AFAIK the update feature doesn't work correctly, but you might want to check with the Firefox Portable support forum about that. I haven't installed Firefox Portable since I did a re-format a few months ago.

Thanks a lot for the quick reply, TonyE! Did not try the sequence you advised yet. Actually installed already the beta4 version in a new location (means have two separate folders within the program files folder) before I asked the question. But when I start the firefox exe in the 3.6 folder, it opens a beta4 version. Is that just because both are using the same profile? Do I need to uninstall Beta4 first or just re-install 3.6? thx