I just upgraded to Firefox 50.0 and I have issues with apostrophes in contracted forms. For instance, I can longer type <<I'm>> with only 3 key strokes because the `m' gets eaten. I need then a 4th key stroke to add the `m'. This issue only happens within Firefox, both in the URL input and in overall text areas. I'm running OS X 10.9.5 and for every other application I tested, the behavior is OK (just as it used to be with the previous Firefox version I had installed). I've tried resetting the browser's locale (en-US) and even my overall system's keyboard/input configuration. However, the problem persists in Firefox. If you'd like I could try something else to help you identify what's causing this behavior, just let me know.