As stated by others above, this is a known issue, with an unknown cause.
Sometimes closing and relaunching FF will fix it. Sometimes you need to change your FF theme. The "fix" seems to vary from system to system. Just find what works for you and use that. I've looked a few times on the Mozilla forums, but since it really only affects a small percentage of users (ie: those using FF and Windowblinds)I don't think it rates high enough to get the attention of the developers.
