Please post a screenshot. http://support.mozilla.org/en-US/kb/how-do-i-create-screenshot-my-problem

In case of any problems related to graphics, the first thing to do is to start Thunderbird in Safe Mode, see Troubleshoot Mode Thunderbird for info. If the issue is gone, most likely an installed theme (or add-on) is to blame which may not be compatibel with a new version of Thunderbird (yet). In that case, you can try updating it, or temporarily disable it until an updated version is released.