Missing form ACTIONs
Say you want to disable a submit button when it's clicked, to prevent the user from submitting twice:
<form name="myform"> <input type="submit" value="Submit" onclick="this.disabled = 'true';"> </form>
On Windows, this works fine in IE but not in Firefox. Or so it appears. What's going on?
Oops, you forgot an ACTION attribute in your form. (It's okay, this is common if you're planning an Ajax-style app.) Without it, IE just ignores the submit click, but Firefox uses the current page as the default action. So it effectively reloads the page, resetting the button state.
The thing is, the submit button is being disabled in FF, just like in IE, but depending on how fast the page reloads, you might not even notice.