https://github.com/tasks/tasks

Open source Available on F Droid. Supports sync through various services.

May be what you looking for or not. :)

What’s the difference with Librefox?

Agree. At first I didn’t like it too much but overall you get used to it moderately fast. There are instances where it fails you and the wrong gesture is triggered, but it’s worth getting rid of the old nav bar.

Give it a try and be patient. :)

An alternative is to switch to gesture based navigation as it would remove the whole navigation bar.

Buy an FP3 or 4 if you don’t need these features.