Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled". Or just hold down the Shift key when you start Thunderbird. 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. If this doesn't help, then please post here your Thunderbird Troubleshooting Information. At the top right of the Thunderbird window, click the menu button (or use the regular menu at the top), then select Help: Troubleshooting Information. Click the "Copy text to clipboard" button.  Do not check the box "Include account names"  Then paste (Ctrl-V), the text into your response here. We only need to see the sections down to and including the "Extensions" section, so you can delete the rest.