You can install other browsers, but the page rendering must be done by Apple’s iOS system version of WebKit. Basically it’s only good for things like bookmark / history syncing.
I know, I was pointing out waterbell's silly argument.
I'm referring specifically to the Mail app on iOS. Other apps, e.g. Wire for replacing iMessage and Mail, will let you choose which browser will open links.
Right, and even then, Apple isn't going to help you at all: links from other apps won't open in Firefox, so you jump through hoops to try to get your Firefox bookmark/history syncing support...
Unfortunately people are better at copying Apple on the annoying shit like this than at the stuff they're actually really good at.
You can't, but you can change your default browser in many apps (such as Gmail).