Because google may stop financing them, and they will never be able to "keep up" since the technical abomination (the word is fair) which is the "javascript web" requires tons of resources. Financing firefox helps google as some sort of anti-trust regulation protection, I don't think the US administration has the intent to regulate Big Tech, a bit similar to the firearms industry. Maybe the US administration is actually totally unable to do it (EU is trying, but it is still _REALLY_ shy).
I recall some firefox dev about how google blink (and google online services) are going into the shaddy/ultra-complex parts of the "javascript web" to break firefox, namely making firefox an significantly less optimal javascripted browser for google services than google blink, or even their brother (apple is a vanguard/blackrock company too) engine, webkit (blink was forked from webkit).
The only way out is all critical(utility/etc in the US) online services which can reasonable be handled by noscript/basic (x)html standard, have to be (like they mostly were before...).
I personnaly do not like firefox neither since it is a c++ kludge abomination, to pathological level, and pulling a c++ compiler makes things even worse (rust syntax is toxicely complex like c++, then this is not a way out, may be worse). No, if you code c++, you are not smart, at best brain-washed , but surely toxic.
