Here's the process to roll back:

First, I recommend backing up your Firefox settings in case something goes wrong. See Backing up your information. (You can copy your entire Firefox profile folder somewhere outside of the Mozilla folder.)

Next, download and save Firefox 3.6 to your desktop for future installation. http://www.mozilla.com/firefox/all-older

Close Firefox 4.

You could install Firefox 3.6 over it (many have reported success) or you could uninstall Firefox first. If you uninstall, do not remove your personal data and settings, just the program.

Unless you have installed an incompatible add-on, Firefox 3.6 should pick up where you left off. If there are serious issues, please post back with details.

Note: I haven't actually tried this myself!

downloaded 3.6 but it wont install. It keeps asking me to resstart computer to complete previous upgrade

Have you tried restarting the computer at least once? There might be a way to clear that without restarting, but it would take a long time to research...

I have re-started many times and still got same response. even shut down for a while too.

Okay, you might need to uninstall the previous version.

Look for Firefox in the Control Panel's Add/Remove programs applet.

If it's not there, try helper.exe in this folder:

C:\Program Files (x86)\Mozilla Firefox\uninstall