I meant to say the "latest Firefox update". I don't think it started with the latest Windows update. I could be wrong.

Firefox 140 has a bug for certain add-on themes, as you can see. I didn't realize it only affected bookmarks. That's weird. Anyway, the workaround for the moment is to change to the built-in Dark theme on the Add-ons page. See: Use themes to change the look of Firefox.

Does that work for your sidebar?

Yes. I followed the approach in Jochen Cezanne's post - my German is rusty; I didn't realize at first that he had the same problem. I had a dark-looking theme (Animated Sea Sunset 02 by Candelora). I went down the list of installed themes until one worked. It is Firefox Aspenglow. This theme looks dark purple for me, maybe because I have the Windows 11 system theme set to dark. All the light themes I have seem to work. Some of the dark themes work, including the built-in Dark.

Thanks for your response