Unfortunately, we have had trouble guaranteeing a consistently high quality of output garbage when input garbage is passed to a jQuery API call. IE9, for example, actually returns the correct element even though the closing brace is missing. Chrome 10 silently returns an empty set. And, as you've noticed, Firefox 4 is the only browser to openly complain in any way about the mess someone dealt to them.

Was there a particular resolution you had in mind for this problem? If we caught the error in FF4 there would still be the problem that this invalid selector doesn't work properly across browsers. I actually prefer the Firefox 4 situation since it actually throws some sort of error to tell you something is wrong.

Wow, that's wild that IE9 returns the element(s)!

My thought was that jQuery would realize that the selector was not in the proper format and drop it right there, not passing it on to the browser at all. But perhaps that's coming out of my ignorance of the inner workings of how selectors are parsed and passed on.

Sizzle passes selectors on to the native implementation whenever possible for performance. It would be too costly to ensure consistent garbage output, as Dave indicated. Fortunately, invalid selectors will not work cross-browser and you can discover the mistake when testing.

Thanks for posting this was very very helpful!