Used to work great however the storage sync limits have now prevented me from adding any additional domains to block.

QuotaExceededError: storage.sync API call exceeded its quota limitations.

Sadly I believe this was introduced with firefox 79 or later that now enforces client-side quota limits.

Would have been great to have a version that doesn't require using the sync storage data if possible however this addon hasn't been updated in over two years so I won't be holding my breath.