Just found out the cause of it- mr. Piro's latest "tabbrowser extensions" Firefox extension is messing up one more time... Surely enough this is the cause, because with TBE disabled Firefox works fine, as well as IE. With TBE enabled the problem exists with Firefox both under Windows and Linux.
Sorry for the false alarm- I will report this brand new bug to mr. Shimoda and see if he will ever make his extension usable...