Found out today that choosing a different User account cures all the problems with windows rendering. All my info is in my regular User account. Please see my post about reinstalling Thunderbird.

in preferences > advance is an option to use hardware acceleration. Change it and see how you go.

Thanks for the suggestion, Matt, but I already tried that to no avail. Interesting that changing to a different User account on the Mac corrected everything about the way the windows rendered. That's why I'm going to reinstall. Only problem is it's a POP account, and I don't want to lose the stored emails.

have you tried safe mode? add-on and particularly themes often do not translate will to new hardware.

I really do not think a reinstall will achieve anything as it will restate system integration, and replace the program files but it will not change any of the settings.

Thanks again, Matt, but I tried a Safe Boot while on the phone with AppleCare. No joy. Any way to remove add-ons? Sorry to be dense, but I’m helping a client and I have virtually no Thunderbird experience. Any and all help gratefully accepted!

What you probably need to do is start TB in safe mode, not safe boot OS X. Help/Restart with Add-ons Disabled, or hold Option key when you launch TB. If the display is normal in safe mode, it's probably a broken-layout problem. Launch TB in safe mode, check 'Reset toolbars and controls', click 'Make changes and restart'.

Thanks to the Mozilla Community for all the help. Disabling the two Add Ons that were active fixed the problem. Windows draw correctly and everything actually speeded up.

mikeyboyca said

Thanks to the Mozilla Community for all the help. Disabling the two Add Ons that were active fixed the problem. Windows draw correctly and everything actually speeded up.

What add-ons were they? It is always good to fear about ones that cause issues. Helps us with the next person to pop up with the same issue.