Obviously part of the 10%. :-( Further research suggests that this may be connected to Firefox and/or Comodo - at least, this has been happening for some folk with these progs on some vBulletin sites for several years. Unfortunately, I've found no cure yet. Frankly, it's such a PITA that I'm visiting this site less and less ...
This site never did what you have experienced under Firefox and Comodo, so i reckon it's something else like a setting you have to change.
Can't help you there, can't replicate the problem.