exploder,
This has happened to me several times with Firefox 3.6.4. And for me, the solution has been exactly as you describe ... restart Firefox. It's certainly inconvenient and annoying, to be sure, since it usually happens at the most inappropriate times. It's niggles like this that are driving me to Chromium and Iron, more and more. It's as if the Mozilla folks are getting sloppy (cocky?) with Firefox, and it just doesn't seem to have the stability it once did.
parnote