After the suggestions that this was a vBB problem and the hints that it may be specific to the new version of Firefox, I've done more testing, and that seems right.
My Chrome works well. Safari is a little inconsistent for me, but I haven't cleared it all out.
But Chrome is doing the right thing for me.
This may have all started when I upgraded to Firefox 5. (Unrelated, I suspect, is that FF5 is deadly with Flash for me - I disabled it entirely.)