Hey! No creepy stuff under the hood :) Substital for Firefox was manually reviewed multiple times by persons from the Mozilla Firefox Add-ons team. It's a pre-requisite to be published here in the store. If there would be creepy stuff under the hood, it would not be published.

About the permissions, we're happy to clear that out for you. The other extension you mentioned is different in many ways. It does not work the same way as Substital and does not have the features that Substital provides. (as you said, it's not that polished). No, Substital cannot work with only the activeTab permission. The reasons is that we inject elements in the page that need broader permissions to be able to work, like for example the area where users can search for subtitles.

To be noted, other popular extensions such as AdBlock or Grammarly also use broader permissions such as "Access your data for all websites".

Hope that helps! :) We plan to add soon a better detailed description why these permissions are needed. Thanks for your feedback!