Did you get an answer yet?

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.)

You can display the classic menu bar and other toolbars/bookmarks in Firefox 11 pretty easily. Press Alt+v to display the classic View menu, then from the Toolbars list, choose the ones you want.

If you prefer, here's the process to roll back:

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

Close Firefox 11.

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!

Please note, Firefox 3.6 will be dropped from support in April of this year. This means that if you do decide to downgrade, you will not receive anymore security or stability upgrades.