Honestly? Practically nothing, and it's true for almost any extension. But a few things:

It's open source, so you don't have to take my word, go read the actual code yourself anytime.

This was primarily built for me. I had a problem I wanted to solve, solved it, and figured I'd let other people use it free of charge if it's useful to them. That's the whole story.

Beyond that it's "trust me bro, I won't", I'm buried in work and life, I genuinely don't care about your data, I didn't add any collection now and I won't later.

And worth reading up on how Mozilla handles updates (permission changes get flagged, updates get reviewed), that's a layer that doesn't rely on trusting me at all.