Yes, why not! Anyone can develop app for the Firefox OS Marketplace and any type of app, browser, photo editor whatever.

If Opera develops for Firefox OS, the users will be able enjoy Opera Mini on Firefox OS too.

The question is very similar to "Would Google (Android) permit Opera to build an browser app for it?"

Firefox OS is a new smartphone OS by Mozilla. It's really a great new smartphone OS and is continuing to grow more and more but sometimes I feel it lacks something and that's Opera Mini!

Opera Mini is the most loved mobile browser I've ever known, mainly because of it's smart data compressing techniques and fast rendering of web pages, the browser that comes with Firefox OS is not that great if you are on a slow network or trying to load a heavy page, but Opera Mini is the way to go.

Any chances of developers of Opera to build a browser for Firefox OS ? That'd be great!