Please write a single answer per comment to clearly see the most popular tools. Vote on the ones you like.
There were too many options in this post and I want to see what are the really interesting ones.
To complement this post:
What file-sharing and media organizing software do you wish that existed?
Qbittorrent is not a browser extension. It’s a gui application.
You don’t install it specifically for our into Firefox.
If you’re trying to use the web interface, just use that. No need for anything specific.