See https://github.com/memeller/shrunked/releases/tag/v5.10.1

All the ones I found, including the one you suggested, either DON'T work with new versions or they are no longer supported. One that works with recent versions of T-Bird and IS still supported?

The one I suggested, v. 5.10.1, works here in both TB 128 and 140.

https://support.mozilla.org/en-US/kb/installing-addon-thunderbird

about:addons This add-on could not be installed because it has not been verified.

Learn more

OK

Are you trying to install in TB or Firefox? Read the support article about installing xpi files.

Thunderbird. Read the document. Was following along when I got the error.

Was trying to add on my LAPTOP. Tried on my desktop and it added like it should. Will keep trying on my laptop. Don't know what the problem there is. Thanks for the help.

See https://github.com/memeller/shrunked/releases/tag/v5.10.1

Thanks for your answer. All worked as posted (of course using both of your links. It's been an issue for me for some time!! Used Image resizer previously but he is actively not supporting it - worth reading his rant on why!! I do understand, though.