It could be that TB is conflicting with some software that is running in the background. That isn't always too easy to work out, though. You could run the computer in safe mode and see if you still get the problem. If you don't, then it would indicate that TB is fine but the problem is as suggested. If that's the case, then close down all the background programs and services and relaunch TB. Introduce them one by one until a launch is affected. Then you'll know which program is causing the issue. It could also be TB itself - but it shouldn't be if it works in safe mode. If it's still a problem in safe mode then I would be inclined to back up all the data files and settings (the 'profile') and, first, over install (but that's a risk). Then, second, uninstall completely, reboot, and reinstall. Finally, uninstall completely, remove profiles, reboot, reinstall and reintroduce the profiles. You would need to relaunch TB after each stage. But don't miss any steps in the help file - https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-thunderbird-profiles - I don't want you to lose any data. And don't try any of this unless you feel competent to do it. Somewhere along the line, you should be able to determine where the problem lies. My bet would be on conflicting software.