Well web browsers are to complex to maintain by part time Dev unless you have an upstream to rebase on or do not mind in implementing new stuff. You need at least 10 to 20 full time Devs at the minimum and that's if you libaries for all web APIs which clearly Firefox and Chrome are not doing. I wish there was a way to browser modular frankly.
The GP referred to "highly-paid executives and HR departments and all that", not developers.
Still. There are some obvious counter-examples of development of extremely complex projects being community-driven under the aegis of much lighter-weight organisations than Mozilla. LibreOffice is an obvious one; The Document Foundation had (at the last report) total expenses of $679,000, with nearly all of that coming from user donations.
To sustain an independent, cutting-edge, competive browser engine you need several hundred excellent engineers working full-time, at least. No-one's been able to do it with less.
No, it's hard to get. Also tricky is what counts as "the engine" varies from project to project. E.g. Webkit doesn't cover as much functionality as Gecko or Chromium, e.g. the HTTP stack is separate.
