I've been enjoying getting involved in Mozilla bug reporting, checking to see if bugs I've noticed have been reported (pretty much all of them have) and reading their status. It's interesting to see the points of contention on how the browser ought to behave, like the age-old debate over tabs in multiline text fields. I personally need a way to enter tabs in textareas (some Wikis use them heavily), though I'd prefer to not interrupt/repurpose the far more useful tabbing between elements. The discussion on what Mozilla ought to do is marred by differences in implementation across various versions of other browsers; some Netscapes, apparently, insert tabs in textareas instead of advancing to the next element, and it could be argued that supporting that behavior is important for previous Netscape users.

Most occasions where it's important to be able to insert tabs into web fields are probably candidates for more sophisticated text editing widgets, though ideally such a widget would have a uniform implementation provided by all common browsers. Not that that's ever going to happen.