2 solutions
There's 2 solutions I'm aware of. It is tooltip related.
1) If you hover over the item that created the tooltip in the first place the white box will disappear.
2) In the location bar type in "about:config" (w/o quotes of course). In the filter bar type in "toolbar_tips" This should limit down to one listing. Double click it and its value will change to false. This will however mean that you don't get any tooltips, but does work.
Hopefully Firefox will get this issue resolved so you don't have to take these actions. Not sure if the bug has been submitted either if someone else wants to check.