How did you come to the conclusion that 750 people is a lot to build a web browser? The Chrome-adjacent teams at Google are about 4,000 people, and that doesn't even include all the people at Google providing infrastructure (e.g. servers, workplace, HR, legal etc.).
Comparing Firefox to Chromium-based browsers doesn't make much sense since these browsers don't develop their own web engine.
take the reference of ladybird.
in a couple of years they built the engine from scratch. it's going to soon enter Alpha. how many people from ladybird built that engine? about 10?
all while everyone has said that modern web makes this task impossible
How did you come to the conclusion that it's not? Google being bloated is not a good justification for why Mozilla should be bloated too. Someone in the comment below suggested that Ladybird was built by about 10 people. Call me naive, but I don't think you'd need 75x number of people to work on a browser that's already established for over 2 decades.