Firefox has been going downhill for a while now, so it's not surprising.
That said, if you don't want to use Chrome, a fairly good alternative would be Vivaldi [0] - a surprisingly neat browser created by the ex-founder of Opera. It's based on the Chromium engine, so no weird rendering shenanigans, but at the same time is quite technically capable, with ad blocking, mouse gestures, email client, etc.
Why? Chromium works well and is easy to extend. It also comes with many security features, includes the latest and greatest standards, has good performance, and is actively maintained.
It lets you focus on creating the best browser possible instead of wasting time working on the parts of the browser engine which aren't relevant to your product.
That said, if you don't want to use Chrome, a fairly good alternative would be Vivaldi [0] - a surprisingly neat browser created by the ex-founder of Opera. It's based on the Chromium engine, so no weird rendering shenanigans, but at the same time is quite technically capable, with ad blocking, mouse gestures, email client, etc.
[0] https://vivaldi.com/