Well, Google, Microsoft and Apple make a few billions a year. Undoubtedly, those three invest a much lower percentage of that into their respective browsers than Mozilla does, but it could still easily be more than Mozilla has.
Also just in general, Google can display an advertisement 24/7 on the world's most popular webpage, and bundle their browser on Chrome OS and Android. Microsoft can bundle their browser with the most popular desktop OS. And Apple can bundle their browser with iOS and OSX, which are both still pretty popular in their space.
Mozilla has none of that, and would have to invest a lot of money to get it. They can afford the occasional billboard and organize the occasional PR event, but that's about it. The entire rest of their user gain depends on them being decisively better than the competition, so that people themselves recommend Firefox over other browsers.
They are the only company to put the user's rights, privacy and best interests first and foremost.
For years they were the only ones to carry the open source flag in the browser space. They had the Mozilla Suite when other browsers were closed, they came out with Firefox which was a blast at the time, and now Servo.
They don't play to a corporate agenda, but tend to do «what is right».
While there is merit to what you say, there's also plenty of places where a divide and conquer can happen with appropriate management of the browser as a whole.
You have the core rendering engine, ancillary features to the application, and the shell of the application itself. From the core rendering engine, you have several areas that are subsets, from svg parsing/rendering, parsing/processing of text/xml/html/css, audio/video, almost all the APIs that the browser offers outside rendering can be implemented by different teams (web-audio, websocket, webrtc, etc).
The communication libraries themselves can be broken off too. There is really a lot that can be handled by many developers.
Now if there were say 15+ developers on any one feature, that is probably too many to be very productive. On the flip side, given the push towards rust, there's room for some duplication of effort there even.
