For the fiscal year 2022 which is the most recent data on Wikipedia, it says that 81% of Mozilla's revenue is from Google (about 480 million) they list 220 million as expenses for software development.
If they lost the Google revenue they wouldn't just have to fire "the C suite" but lay off most of their engineers. An independent browser engine is a project with code in the tens of millions of lines, you obviously need to pay hundreds or of engineers to work on this, which is why there's pretty much only three competitive ones, all maintained with significant resources.