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

I am having the same Issue!. Windows 7 64bit Uses 90 some percent of my processor , crashes repeatedly, windows freeze. Hate it! 4.0 is a Useless peace of crap!

Firefox 4 is a complete nightmare. I cannot do ANYTHING on my computer with my browser switched on, and I'm considering taking legal action against Mozilla for foisting this utter crap on people.

I CANNOT WORK WITH FIREFOX 4.

I NEED A SIMPLE ONE-CLICK DOWNGRADE.

THERE MUST BE ONE AVAILABLE SOMEWHERE.

I just found that I cannot log into a website I'm a member of. Wrote to their admin but they have no problem with the login, so it is the browser (Firefox 4.0) that is preventing me from logging into the site, or so it seems.