Firefox is fine. The people running it are not 1 year ago
Firefox code is not fine. It is 25 years old code, with many stuff bolted on top (multithreading). It does not even have a proper security sandboxing for renderer!
This codebase was underfunded for a very long time! And all rewrites and major refactorings were cancelled!
Nobody embeds Gecko engine anymore. There are good reasons for that!