I was using Firefox 3.5 when this happened.
I tried the obvious workarounds: Upgrade to 3.6. Downgrade to 3.0. None worked.
I'm using Windows Vista 64-bit.
The only extensions installed are Lori, Roboform, Google Toolbar, Live HTTP Headers, Skype, and Yahoo Toolbar -- all of which I have used for a very long time with no problems.