Yes, they can work on many things simultaneously but it is worrisome when the execution of their core product fails [1] it shows a big management failure.
[1] https://www.google.com/search?q=firefox+market+share&oq=fire...
[1] https://www.google.com/search?q=firefox+market+share&oq=fire...
Man, I wish that were my experience. I'd love to keep using Firefox. My experience is the opposite of that, though.
IMHO there is a room for innovation in this space, long term but you can see if this happenning observing the project. Currently browser engines are one of the most complex pieces of software. There are new ways to attack these mammoths.
The amount of Rust code in Firefox is pretty substantial, and growing. Did you expect an instant rewrite?