I just did a test on my Windows 10 computer for changing Windows' setting for the thickness of the cursor. Even after a reboot, it seems that that setting doesn't apply to Thunderbird, Firefox, or many of my other programs. So that solution is out.

Before I get into more complicated matters, I'll ask this. Does text on your screen show up as fuzzy? Or clear? If it is fuzzy, then perhaps your monitor resolution is not set properly. In addition to fuzzy text, the cursor might be fuzzy enough to not be seen.

Thanks for your help so far. The cursor is fine in Word and other programs it does not show up only in Thunderbird.

Is there some setting in Thunderbird?

Sorry - forgot to tell you the text is very sharp and clear.

I think there is a bug in Thunderbird. I see the same problem, intermittently, but it can normally be fixed by switching to html source view and back to the Edit screen.

I'm using the Stationery addon which introduces a tab for the source. Toggling between the tabs for source view and regular message view does the trick. But I haven't been able to pin down the precise sequence of actions needed to trigger this fault.

I now see that the cursor is back so you are correct that it is intermittent I will try and change the view next time to see if this helps.

just to be technical, that blinking bar is the system caret. Windows only has one, that all applications must share. Like Zenos I think we have a bug, somewhere in the getting it back part of the program. But I digress. This is the bug I consider it to be. https://bugzilla.mozilla.org/show_bug.cgi?id=226301