FWIW: Firefox 15.0.1 is the current version :-), 14 is old.
Aside of that, webkit powers both Safari (on mac and on iOS) and Chrome (all platforms) finding differences between the two means you need to go very on the edge of things.
Opera and Firefox use different engines, but all of them adhere pretty well to the standards, and bugs that are reported typically do get fixed.
IE OTOH: it's not adhering to standards, or if it does the standards are ancient by the time they get adhered by the folks in Redmond. As such they only evolve for security bugs or patent infringements. All other bugs - even if they get so well known that they get named - remain unfixed forever. To add insult to the injury there are the widely publicized blocker settings to prevent machines from forcing an update - widely deployed by corporations.
Compare that to e.g. chrome: it updates itself to the most recent stable version - and it "just works". Firefox seems to be ready to do the same updates, even if it's declining in market share that's a good thing in itself.
We really need an IE tax ;^)