I don’t think safari is even remotely comparable given that it’s a default browser on macs.
How so ? The default browser on Windows is Edge, people keep installing Chrome? Chrome is available on MacOS, yet people stick with Safari?
some people stick with safari, but no one is replacing chrome, fire fox, or edge with safari. People choose to replace edge because it is obtrusive and annoying to use, safari isn’t.
In that context, safari is not a competitor for Firefox in the same way chrome is. It’s comparing apples to oranges.
Also, I’m pretty sure it’s not possible to install any other browser on iPhones unless you get root.
Edit: It looks like you can with iOS 15.0
As I understand it, any browser on iPhone has to be built on WebKit, so even if you install fire fox or chrome, it’s running on a totally different web engine than the desktop version. Making them more safari re-skins in the same way that stuff like brave or opera are just chrome reskins.