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.
How about, if you want a broken version of Firefox, you compile it yourself, rather than let everyone else suffer?
Like, the vast majority of browser users don’t even know what an extension is, let alone install one.