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.

All went as described until installing Firefox 3.6 (had do uninstall 5). Attempts to install failed with notice "Can not open output file."

You might have gotten a bad exe file. Can you download again?

I assume when you download you are using Save rather than Run. You also might want to right-click the download link and use Save Target As.

This is another source for the download: http://releases.mozilla.org/pub/mozilla.org/firefox/releases/latest-3.6/win32/en-US/ (if you prefer another language, navigate up a level to select it)

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With some older versions of Firefox, it also was suggested to empty the Windows Temp folder, since the installer may be having trouble unpacking the download in there (file name conflicts perhaps??).

Thanks so much for this thread, it solved my problem! Easy fix, yea :)