UPD: The issue has been fixed in version 9.9.23
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Hi

Thank you for the message! Sorry about it. Looks like the bug in the latest Firefox version. I've prepared a new version of 9.9.23 and am awaiting approval from the Firefox team. It takes several days. I'll notify you after it will be available.

Temporary solution: open the page: https://addons.mozilla.org/en-US/firefox/addon/youtube-subscription-groups/versions/ and click 'Download file' on version 9.9.18. After that, it'll be working.