VERY useful extension for managing a large number of subscriptions. I ran into an issue a few weeks ago where my Subscriptions list was empty and couldn't see or add new subscriptions to a Group using Firefox. The issue has to do with Firefox's built in Content Blocking. To fix the issue, here's what I did:

Go into Firefox's Privacy & Security section and do the following for Content Blocking settings:

Send websites a “Do Not Track” signal that you don’t want to be tracked: ensure "Only when Firefox is set to block Detected Trackers" is selected.

Third-Party Cookies: ensure "Trackers (recommended)" is selected.

Changing these two settings fixed my issue and now the extension is working great again. As a side note, I am using AdGuard AdBlocker extension to block ads and such in YouTube and this adblocker does not appear to be affecting this extension at all...