I often talk to people reporting a broken website in qutebrowser where it turns out to be due to a Firefox user agent.

Some examples (there are lots more via IRC/Reddit/...):

https://github.com/qutebrowser/qutebrowser/issues/4302 https://github.com/qutebrowser/qutebrowser/issues/3822 https://github.com/qutebrowser/qutebrowser/issues/1187

It can be the result of programming in the style of

> if(browser == firefox) {new_feature_chrome_does_not_support()} else {old_feature()}

instead of

> if(new_feature in navigator) {new_feature()} else {old_feature()}