Crashing browsers: hmmmm ..... Explorer is, of course, the easiest browser to crash. Any fool can crash Explorer. Just open up lots of windows, and it will fall over every time. Especially if they are graphics-heavy.
Next easiest? Hard to say. I actually crash Mozilla next most often, but then I habitually push Mozilla out to a truly ridiculous number of tabs and windows. Right now, by my standards, I have only a modest amount of stuff open: er .... 13 Mozilla windows and ... (longer err) 97 tabs, but that's because I spent a few minutes shutting down another 50-odd tabs a little earlier. It's not unusual for me to have double that number going. And now and again, maybe once every two or three weeks, Mozilla cries uncle and dies on me.
Opera is my next most-crashed browser. I crash Opera about once every three months. But then I typically only have 2 or 3 Opera windows running, with (guess) 30-odd tabs in total. (Once in a while lots more though.) And when you crash Opera - well, when the power goes off on my desktops, or when I stupidly press the power button on my docking station when I meant to press the undock button (bad design, IBM!) , actual crashes are too rate these days to really count - Opera takes me straight back to where I was before. (Great design feature, Opera Software.)
My least-crashed browser is Firefox. But then my current Firefox workload is typical: zero widows with zero tabs. When I do fire up the fox, it rarely goes beyond a dozen tabs before the dumbed-down-for-the-unwashed-masses user interface starts bugging me enough that I shut it down again. Mostly I only use Firefox when I've got so many Opera and Mozilla windows open that it's hard to tell which one is which, and i just want a browser that stands out in a different colour on the task list for a few moments while I take care of some small job. Oh, and I also use it when I want to test some web page that has external Javascript on it that itself depends on a fixed-at-browser-startup variable, typically screen resolution. To test pages that do screen-size sniffing with Javascript, you have to code, test, switch resolutions, close down the browser, test, and so on. I.e., you need a browser on your machine that you aren't using for anything else in particular so you can shut it down and restart it everytime you want to check that your code is working properly. Firefox serves that role admirably, 'cause while it actually renders web pages properly and according to the standards (like Moz and Opera, unlike Internet Explorer) the UI is too crappy to bother using it for anything important.
Which would crash least often if I used them equally? Don't know. It's reasonable to assume, in view of their common code base and pending evidence to say otherwse, that Moz and the Fox would be the same. Better than Opera 7? By a mile. Better than ancient old Opera 6? Easily. Better than Opera 8? Don't know. Neither one crashes often enough to ever get excited about. I mean my habitual usage probably should be called "abuse" not "use", and even so I don't crash browsers often enough to ever worry about anymore. So who cares?