But they both have URL bars, copy!
When Firefox came out, they wanted to differentiate themselves from the clunky and awkward cross-platform UI of Seamonkey, so they emphasised following platform conventions.
When Chrome came out, they wanted to differentiate themselves from existing native (and native-feeling) browsers by being minimalist, so they invented their own cross-platform UI. Some people were quite happy to learn a new UI to gain perceived speed and switched to Chrome, some people preferred native-looking apps to Chrome's gratuitous differences and stayed with Firefox.
Then Chrome started eating Firefox's marketshare. Whether that's because of better marketing, technical superiority, or just technical differences in ways that happened to matter at the time doesn't matter; the point is that these days when most people think of "web browser" they imagine something that looks more like Chrome than, say Firefox 3. And so Mozilla feels that to stay relevant, Firefox has to hide its menu-bar, put tabs above the address bar, cram a whole bunch of disparate things into a single hamburger menu, and so forth.
There's a small but vocal contingent who stayled loyal to Firefox and avoided Chrome so they could avoid those UI idioms, and the fact that Firefox is now adopting them anyway feels like a betrayal of sorts. I'm not saying it is anything melodramatic as betrayal, but people are entitled to their opinions.