Set skins to default ones in Firefox and that mail program from Mozilla... Firebird?
The problem is related to skins for Mozilla products, some of them aren't working properly.
Just a hint that worked for me. Check if in the Status Bar there isn't too much programs (extensions) and buttoms.
If the width is larger than the screen, Firefox try to adjust, the screen jump, and, after all, not all the toolbars fit in the screen.
Well, remove the extensions that make the status bar larger than the screen.
I hope McAffee SiteAdvisor team listen my asks for narrowing the long unlegible buttom they add in the status bar