Firefox compatibility is valuable because Firefox extensions don't have to be distributed through the Mozilla add-on store (they do need to be signed by AMO, but provided your extension isn't doing anything illegal, that should not be an issue).
Finally, you could try redistributing unbranded Firefox or Chromium with your extension pre-installed. Waterfox (a Firefox fork) can have DRM — it's disabled by default, but it can be switched on — and I don't think they put a great deal of effort into it, so I think that your "version" of Firefox could also easily have DRM. (I have no idea whether the same holds for Chromium.)