Due to security vulnerabilities, rolling back to Firefox 4.0.1 is not recommended. Firefox 3.6.18 would be the safer choice.

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 5.

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!

If you need an older version of Firefox for just one website, it would make more sense to install a Firefox Portable 3.6.16 version to use there. It won't affect any other version of Firefox you have installed. The Portable version installs to its' own set of folders, eliminating the need to "backup" anything or be careful when un-installing the regular version of Firefox. Plus, when you are done with needing it, it can be removed without affecting anything else. BTW, Firefox 4.0 & 4.0.1 aren't supported any longer, 5.0 was a security update for those versions. Firefox 3.6.18 received the same security fixes that are in Firefox 5.0. They are the only two versions of Firefox that are currently supported.

http://portableapps.com/apps/internet/firefox_portable/localization#legacy36