As noted elsewhere, Microsoft does not allow alternative browsers on Windows Phone 8.
UC Browser and the others are merely skins on top of IE10/Trident. So you can get different menus and buttons, but the actual rendering engine is the same.
Until Microsoft changes its policies (not likely), you won't see Firefox or Opera or Chrome or any other actual browser on WP. (For what it's worth, Apple has similar restrictions in place on iOS -- Safari/Webkit only.)
The best that can be done is something like UC Browser's "speed mode" (which is terrible -- try it) or Opera Mini (which is a lot better, but not available on WP), both of which render pages remotely at the respective operator's servers and send a compressed binary for display client-side. There are a lot of very noticeable trade-offs to this approach, and it's not suitable for a primary browser.