You don't say what you have running as you come out of Hibernate; my guess would be Firefox is one as this is not an uncommon occurrence with either the browser itself or an Add-on - but I have little idea what actually causes it!!
I do know that the problem can be resolved (sometimes) by either reinstalling Firefox or creating a new profile. If you create a new profile make certain you transfer your bookmarks before you delete the old profile or you will lose them; here's how link text. You should also make a note of your saved Passwords, user names and the sites they apply to - just in case.
Mozilla also offer some advice that may help link text