I can only give you my personal experience, becasue you'll have to try both to see which is better for YOU.
All these answers are "for me":
Opera loads pages a bit faster. Of course I don't have my FF loaded with addons, so, your mileage may vary.
Opera has spell check, it will underline questionable spelling as you type. It will not suggest corrections (that I am aware of). FF has spellcheck as well.
Firefox has by far more customization abilities. I find Opera is great "as is" , but many who try both FF and Opera will decide on FF because of the many addons.
Again, for me Opera is steady as a rock. FF has bugged out on me requiring a restart, but it's not to be considered buggy. And again, this may vary if you run a ton or addons.
Both should play just fine with Kaspersky.
Opera is my default browser, so definitely give it a try, but most will answer FF as it has a much greater user base.