I believe Firefox objects when the maxversion is higher than any existing version. Try setting it to 15.* instead of 20.* and see whether that makes any difference.

It no longer makes a difference now anyway. Firefox is accepting those themes after I reinstalled them. You could be onto something, however. Halloween was set to max version 10.*. That was the older theme with no problem. I thought Firefox would not accept themes with a max version less than current?

That does not explain why Sky Pilot was not rejected as incompatible. Its max version is set to 20.*. There has to be some other factor.

Due to public outcry, Firefox stopped disabling extensions with an old maxVersion as of Firefox 10 (see Firefox 10 for developers - MDN). However, Firefox will enforce maxVersion if you add a strict compatibility requirement to your manifest.

So the other factor is the manifest. Why does that make no difference

when the add-on is reinstalled?

I think you might try the AMO forums for better answers... https://forums.mozilla.org/addons/

It would seem that I cannot log on there. The instructions for logon say, "Log in below using your Mozilla Add-ons account username and password.". That does not work. I am told that I gave an incorrect username, and it is the same as what appears at the top of the page for an addon. I guess that will have to remain a mystery unless someone at another forum knows.