Comment by Jotalea
I've been using Firefox mobile browser on my not that old Samsung Galaxy S21FE for one year or more, and I can confidently say that the performance is awful. I often see delayed app loading (the navigation layer loads while the actual webview is still transparent) [1], it has a high memory usage and often caused the system to terminate another app that runs in the background at the moment. Rendering issues are also a thing. It often glitches and flashes white when opening/closing the on-screen keyboard and it blinds me for a second if using at night.
But I keep using it because I use the sync feature and it sucks less than chrome.
On my 3 year old Samsung Android phone Firefox mobile works fine and I don't have any issues. Try reinstalling the app or restarting your phone.