you probably do not have a default account defined. click 'account settings' and set one.

Thanks David. I had a default account defined. I have four different accounts all had the same issue. The problem was solved with the following:

Thunderbird not opening compose windows (new message, reply, forward) This issue, where clicking on "New Message", "Reply", or "Forward" in Thunderbird doesn't open the compose window, is a common problem with several potential causes. Here's a breakdown of troubleshooting steps to get you back to sending emails:

1. Restart Thunderbird in Troubleshoot Mode If Thunderbird is running, click ≡ > Help > Troubleshoot Mode... and choose Restart. If Thunderbird is closed, hold down the Shift key while starting Thunderbird. In the Thunderbird Troubleshoot Mode dialog box, click Continue in Troubleshoot Mode. If the compose window opens now, it suggests an Add-on or Theme might be interfering. You will need to re-enable add-ons one at a time until you locate the offender.

Hitchhiking on this thread, after updating to Version: 143.0b1, I suddenly have the same problem. I checked the solution and complied but the problem persists. Is it possible to rollback the update?

I can't duplicate the problem in 143.0b1, so you might try what the prior person wrote. I never trust betas except for testing.

Found a solution in Reddit. I'll walk through it here.

The first response was this: Press ALT-V then T then M. Then click View > Folders > Folder Pane Header.

That didn't help me because I didn't have any themes or add-ons enabled.

Next up was this: "Your first step when facing a new problem, almost always, is" Help > Troubleshoot Mode. See https://support.mozilla.org/en-US/kb/troubleshoot-mode-thunderbird

I replied that that was the first thing I'd done. I was then asked if the problem cleared up in Troubleshoot Mode?

Yes, it did but once back in regular mode the problem remained.

The suggestion that worked for me was this: "Open the profile folder - TB menu > Help > Troubleshooting Information and then under the Application Basics section next to Profile Folder, click 'Open Folder' and close TB. When TB is closed, rename xulstore.json to xulstore.jsox and restart TB"

That suggestion fixed my problem. Apparently, the xulstore.json re-writes itself and thus far anyway my data seems intact. I will revisit this page if there's anything new to add.