Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled". If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender.

Thank you that fixed it. I enabled all the extensions one at a time but now none of them are causing the problem.

Unfortunately we are seeing that a lot. and no one knows why.