Since bug 456535, the bookmarks toolbar expands 1 px when Menu Bar is set to show (decreases 1px when the Menu Bar is set to hide) STR: 1) Toggle the Menu Bar to hide/show and watch the tab bar move down a little but it is from the bookmarks toolbar expanding by one pixel.

I botched the STR. 1) View->Toolbars, uncheck 'Menu Bar' entry 2) Hit the alt key 3) View->Toolbars, check the 'Menu Bar' entry and watch the tab bar move down one pixel. The tab bar isn't actually the toolbar that is expanding, it is the bookmarks toolbar expanding by one pixel

The naming of this bug is not really accurate, because it also happens with the bookmarks toolbar disabled. So it probably is the box the menubar, navigation bar and bookmarks toolbar reside in that changes by 1 pixel.