That image does not look like the standard Thunderbird theme. It looks like the theme you are using somehow has white background and white text in the 'Menu Bar' and 'Composition Toolbar' area.

You seem to be using either 'userChrome.css' file OR you may have downloaded a theme. What theme are you using? Whatever theme you are using swap it for thunderbird 'Light' default theme and then report on whether you can see that missing toolbar. Thunderbird is not responsible for any theme you download and use. So if this is a theme you have downloaded then you need to contact the author of that theme and tell them about your problem.

Maybe the theme is not suitable for the version of Thunderbird which you are using. What version of Thunderbird are you using?

Thanks, but I am not using a theme with Thunderbird. I stay away from themes and most add-ons because I know that they just add complexity.

The problem isn't theme related. The code that generates the composition screen must be corrupted.

I suspect that my next step is to re-install Thunderbird. Now, how do I do that and be certain that my mail files are not disturbed?

I did mitigate the problem by downloading a very hard to read theme. But at least it's usable.