it works fine. The issue may be that you have other software that intercepts the shortcut.

I'm running Windows 11 with Malwarebytes, MS Office 2010 and Firefox. I can't think what could be interfering with only that Keyboard shortcut and only in Thunderbird. It all worked fine on my last machine running Windows 10. Any ideas? Even a different set of keystrokes would be better than having to reach for the mouse every time I wanted Bold text.

If there's other software intercepting the shortcut keystrokes then it's not apparent as nothing else seems to happen when I try to use those keystrokes.

Does the same thing happen in safe/troubleshoot mode (hold Shift when launching TB)?

Yes, the same thing happens in safe/troubleshoot mode.

By the way, thanks to everyone for such a prompt set of replies for such a trivial problem. It would be nice to find out why this is happening and how to fix it, but I'm not in a rush, and I appreciate that you are all volunteers.

It seems to be a common problem, sometimes tied to settings in Office or Chrome browser.

I don't know a general fix, (it works on my setup with neither Office nor Chrome), but there is a workaround using the tbkeys-lite add-on. In the attached picture, I've set Alt+B as a shortcut for bold text, in tbkeys settings. The picture also shows Alt+Z as a shortcut for strikethrough.