We are seeing the same problem, even in safe mode with all add-ons disabled. Windows 7 Professional 32 bit.

This can be a problem with the file places.sqlite that stores the bookmarks and the history.

• http://kb.mozillazine.org/Bookmarks_history_and_toolbar_buttons_not_working_-_Firefox

Create a new profile as a test to check if your current profile is causing the problems.
See Basic Troubleshooting: Make a new profile

If that new profile works then you can transfer some files from the old profile to that new profile (be careful not to copy corrupted files)

See http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

Thanks for the tip, but that did not work. I uninstalled FF4, including deleting everything in Program Files that was not removed. I created new profiles, I even tried removing all JSON backups and the places.sqlite file. Still, the buttons do not ever become enabled.

I've done all of this as well.Tried deleting the places.sqlite file,didn't work.Did a fresh,new install of FF4,without any addons,that didn't work,either.I ended up going back to FF 3.6.16 until they get the kinks worked out.I have to have my back/forward function.(I'm using Windows XP.)

Same as the above users. I have found that it comes and goes. For example, if I reload a tab, sometimes the back/forward buttons will return across ALL tabs, not just the ones I reloaded. But this is hit or miss. Looking forward to a fix/workaround!

Edit: Also, FYI, I'm on Vista 32 bit.

FWIW, I know OP did not have any add-ons enabled, but what seems to work for me has been disabling McAfee SiteAdvisor as per this discussion on the loss of back button functionality:

https://support.mozilla.com/en-US/questions/753639#answer-150669

Again, apologies that this workaround won't work for the OP, but I know in my case I disabled/reenabled my add-ons one by one and sure enough, once I turned on McAfee again, I lost the back button.

Ive upgraded to Firefox 4and I am VERY DISPLEASED with it, to put it mildly. I wish there where some way to reinstall to the former version. I'm all for fixing bugs etc... But some things are better off left alone. I'm so unhappy with this upgrade I seriously considering uninstalling Firefox all together and I've been using this browser for over 5 yrs. I'm all for improvement . But sometimes its best to remember the say ' If it ain't broke , don't fix it'

I encountered this problem after installing Audacity, although I'm not sure if this was the direct cause. After running safe mode I found my Back/Forward buttons worked again, and after re-enabling each add-on one by one, I identified the Yahoo Toolbar as the problem in my case. Hope this helps.