If it's not already there, will you try adding this option
to your device section of xorg.conf to see if it helps or not. Honestly, it doesn't resemble the usual problems but you never know.
Thanks for the suggestion. Didn't help, unfortunately. Still since no one else what seeing it, I started wondering about a profile problem. And realized that I've been sharing the profile between VL (6.0 Standard, Light and SOHO) and Salix_64. Which tends to explain why I was seeing the problem in multiple distros.
Renamed the existing profile, fired Seamonkey up again, problem gone. So it is something that has got screwed up in the profile.
I used to have some notes on migrating things like passwords and mail to a new profile, but so far, haven't managed to find them. Does anyone know the required steps? I would hate to lose either the passwords or the mail. I've got the passwords stored elsewhere, but it would take forever to enter them all, and I really don't want to lose the mail.