Hello!

Could you remove the speller and readd it?

There was a Thunderbird update that disabled the speller, not sure if it also happened with Firefox.

Also, notice that Mozilla (Firefox) automatically changed all the spellers add-ons to WebExtension.

This means, there are now two dictionaries:
1) Firefox: WebExtension;
2) Thunderbird: Legacy extension.

I believe that 1) won't probably work with older versions of Firefox.

If you have any issues reinstalling, please follow the guide:
http://proofingtoolgui.org/en_installing.html

If it doesn't solve the issue, please send me a private e-mail to try to see what happened.

Thanks!

EDIT: It is always a pleasure to help.