The BackBerry OS's like BlackBerry 10 and the older BlackBerry Tablet OS for PlayBook is based on QNX. Once long ago you could build Firefox for QNX but support for QNX has long been removed from the Mozilla source tree.

There has been no development of a mobile browser from Mozilla for any BlackBerry OS or hardware.

Mozilla only has Firefox for desktop Windows, Mac OSX, Linux and the mobile Firefox for Android 2.3+. There is also "Firefox OS" used on say some budget phones and certain new Panasonic Tv's. They also just recently started with a Firefox shell browser for iOS 8.2+.

Firefox for Android runs just fine on Blackberry OS10. You can't get it via the Google Play Store, but you can download the apk file and install it. Download the apk from https://wiki.mozilla.org/Mobile/Platforms/Android

Note, you'll need to do that every time an update is available.