This one’s an interesting read,

“The New Browser War: Mobile Firefox vs. Opera Mini” is the title.
Vaporware vs. Currrently available and pretty capable browser, capable and multi platform(java midlet), well, I could enumerate many of its features, but the thing is, one exists, the other doesn’t.
They’re also on different categories, Mini’s a lightweight browser, it doesn’t even render the pages itself, like SkyFire, rendering’s done by a remote server, it also doesn’t support javascript(processed by server) & plugins, unlike Opera’s other mobile browser for Symbian & Windows Mobile: Opera Mobile 8.65, which is in the same class as Mobile Firefox, Mobile Safari, others.
Well, but moving on:
Why Firefox Might Win
Athough Opera Mini is a great mobile browser with tons of features, back on the desktop, Firefox has more users. So, when it comes time to choose a mobile browser, the one that syncs with your desktop browser of choice is likely to win out. Plus, if add-ons are developed for Mobile Firefox, it has the possibility of becoming a killer mobile app.
In my case:

Yeah, when I started using Opera Mobile(pushed to me by Sony-Ericsson IIRC) on my P800 back in 2003, IE was my browser on the desktop(dear lord), and it had and still has much more users than Opera or FF, unfortunately.
After that, I was so pleased with Opera Mobile that I decided to give the Desktop version a try, it became my desktop browser, a few years later I switched(not completely) to FF but only because of my work( benefited a LOT from the extensions ), and a few compatibility issues( the browser’s not to blame, the web devs are - the ones at google(mail) for eg. ).
Now, I know not everyone would follow this path, usually nobody goes looking for a new browser, same way nobody goes looking for a new/different OS, actually, as I stated, I didn’t either, Sony-Ericsson did it for me, but what I mean is: regarding browsers, people follow the ‘if it isn’t broken, don’t fix it’ rule.
“Mobile devices are a different ground”
They are, Opera’s already the standard on a couple of WinMo devices and lots and lots of other/different devices - sometimes the user isn’t even aware of it, like with the Wii, and other times, like with Opera Mini,

Above is the translated page(click to zoom) for a service provided by a Portuguese mobile operator: Optimus(owned by Orange & Sonaecom).
Optimus hasn’t got the largest market share in Portugal(20%), but Vodafone PT does, and they also reference Opera Mini in their mobile internet service FAQ, not sure of the other major Portuguese Mobile Operator, TMN, but I believe it follows the same behaviour.
Like these Portuguese mobile operators, many others are also promoting the use of Opera Mini to their customers, which is great for Opera since it generates lots of brand awareness, users.
This isn’t strictly related to the ‘mobile browser realm’, but if Opera ASA is smart, they’ll take advantage of their cool service, Opera Link, to bring mobile users to the desktop too. Perhaps by giving special emphasis to it in the first usage wizard of Opera Mini/Mobile/Embedded.
Addons Schmaddons
Addons are indeed a major advantage of the Desktop Firefox, some(like me) may say that without them Firefox is useless, but the majority of people who know what Firefox is isn’t even aware of that functionality(really). Firefox attained its ~16% market share being user-marketed as a secure(lol) and fast browser, took advantage of the ‘windows and internet explorer is slow, unsafe, will eat your children’ and hip-because-i’m-different factor.
I don’t believe that will be an advantage on the mobile world, to start, the usage pattern is different, users won’t be using the browser for extended periods of time,