Visit about:compat in your firefox. I find it insane that these exist.
Edit: I’ve learned that this is part of the webcompat system addon developed by Mozilla and other contributors. I see why this is beneficial default behavior, since FF has no chance of getting enough market share to matter more if things are broken.
However, this behavior is too intrusive for my taste. For example this injection: https://hg.mozilla.org/mozilla-central/file/8a4afb4d34f8/browser/extensions/webcompat/injections/js/bug1472075-bankofamerica.com-ua-change.js is basically just to silence annoying user reports.
Also, Every site FF pretends to be a different UA on is artificially reducing FF market share data.
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 ).