A quick search turned up this for me: https://blog.mozilla.org/firefox/the-new-firefox-by-the-numb...
It's a bit dated, but at the time of the Quantum release,
> How many authors contributed code?
> More than 700 authors contributed code to Firefox ...
and I guess the majority of those code authors were working for Mozilla.
It seems unlikely there's been a 90% reduction in the engineering effort being put into Firefox in the last couple years.
There's also this gesture of assuming that the opponent is motivated by evil intentions, which is almost always wrong.
On topic though: Our conversation could really benefit from some actual numbers. I made a quick search but didn't find anything.