I've a feeling that I may be way down the list of people who already know this, but there may be some left who don't.
I've just discovered that you can use Stumbleupon with enough of the features working, without using Firefox and the Stumbleupon extension.
I can log in and all my favourites are now usable in Opera.
I use Opera 99% of the time now, and haven't thought of "stumbling" in an age. That's a great improvement, to use it with Opera, or any browser I assume.