It initially worked fine, but something changed that makes it so it doesn't work without reloading the subscription page. That was the whole reason why I picked this and didn't just write a tampermonkey script, because it is exceedingly annoying to have to reload the subscription page every time I navigate to it.

Edit: You can say it shouldn't but that's what happens, even after all other extensions disabled (including uBlock Origin, which I wouldn't be okay with disabling just for this functionality anyway). Seemed to work before, as I indicated, but either Youtube updated something or the latest update broke it. Firefox on Linux (Pop!_OS) version 147.0.3, though I don't know how much that matters.

Edit2: This seems to be fixed, thank you!