Edit: great to hear that it works now and thanks for updating the review. PDFs are currently not supported, unfortunately.

I'm sorry to hear that the extension doesn't work for you an. I'd really like to understand the cause to further improve the extension.

If you'd like to, please reach out (contact@mark-my-word.app).

A few interesting data points would be:
1) Which version of Firefox are you using
2) Which other extensions are you running alongside Mark my Word?
3) Are any errors or warnings in the console (press F12 and check the "Console" tab)?