Maybe:

• http://kb.mozillazine.org/Corrupt_localstore.rdf

Can you be more specific? I don't see anything there about temporary changes, just permanent ones.

I have tried checking some of the boxes on that "Firefox Safe Mode" dialog, and lost a lot of information: NoScript forgot all its exceptions when I re-enabled it, my customizations to the toolbars had to be redone, etc. I did everything except resetting the bookmarks and search engines, and it didn't help. But Safe Mode fixes the problem.

Not running in Safe Mode - disable your extensions one by one, or in groups if you have a lot, until you find a conflict or culprit.

You could also try disabling Hardware Acceleration - Options>Advanced>General>Browsing> uncheck Use Hardware Acceleration ...

Safe Mode doesn't affect plug-ins such as Flash but make sure they are up-to-date.

Thanks, it was the hardware acceleration that was the culprit. Turning it off fixes things.

Is it worth submitting a bug report about this?

No, it is a well-known problem that is related to the user's set-up, perhaps with the graphics card not being up-to-date, too old or inadequate. This is not specifically a Firefox problem. Start by updating your graphics card.

This is not my field at all. You could try Hardware Acceleration in the Search Firefox Help box above but I would recommend you try a new question, say, <My Fx4.0.1 doesn't operate unless I disable Hardware Acceleration. What is the remedy?>

This thread is starting to look a bit old and will get buried.