Agreed. To expand on your OS comment, SerenityOS is an operating system that was largely written by one guy. Then he started a web browser for it ( Ladybird ).
Despite having a lot more help on the browser, he expects it to take longer. It is very clear that a modern web browser is a much bigger undertaking than the OS.
A browser engine is such a significant investment that even Microsoft sees it as too much effort. They dropped their internal engine to switch to Blink ( Chromium ).
If they are having to add compat, it is because it is a popular site that is already ignoring Firefox. I am sure they have communicated the problems. The website operators don’t care.
What hurts Firefox market share is when regular users have problems on the sites they frequent. The lower Firefox market share, the fewer sites care about it ( as you seem to understand ). Firefox has to make these kinds of fixes.