To downgrade to Firefox 3.6 first uninstall Firefox 4, but do not select the option to "Remove my Firefox personal data". If you select that option it will delete your bookmarks, passwords and other user data.

You can then install the latest version of Firefox 3.6 available from http://www.mozilla.com/en-US/firefox/all-older.html - it will automatically use your current bookmarks, passwords etc.

To avoid possible problems with downgrading, I recommend going to your profile folder and deleting the following files if they exist - extensions.cache, extensions.rdf, extensions.ini, extensions.sqlite and localstore.rdf. Deleting these files will force Firefox to rebuild the list of installed extensions, checking their compatibility, and reset toolbar customizations.

For details of how to find your profile folder see https://support.mozilla.com/kb/Profiles

What problems are you having with Firefox 4?

I appreciate finding this information. I was just about desperate to get rid of FF4 because of the issues it gave my old/slow computer. It used huge amounts of CPU and either hung or crashed repeatedly. I had to restart it over and over to get it to work at all, only to have it seize up again a few minutes later. I read the help files here and tried everything it said. Nothing worked for me. I hope that there will be some sort of patch/fix that will come soon, so that I can upgrade. I do not wish to be forced to use IE, and I can't stand Chrome. Thanks again for the help.

Add ons that not supporting mozilla 4 was the problem... For this reason i decided to...return to mozilla 3.6

I am also trying to get rid of 4, mainly because my add ons won't work. It's been really frustrating.

I have a slightly different problem. Whenever I type a new address into the URL bar and hit enter, nothing happens. Instead of a reload button, there's a green arrow. I can't go to the new address unless I stop typing, take the mouse, and click on the green arrow. That's just ridiculous. I want the enter key to work again. How do I fix that?