I made my first contribution to Firefox the other day... I downloaded the source on a whim, realised I could build and run it - so went to the bug tracker.

There was a bug with the box model editor that if the labels were long (such as "auto") then they would overlap the borders. I made it so long labels will rotate 90 degrees (try it out, change the outside margin to "auto").

I'm pretty stoked that some code I wrote is in Firefox - seeing an advertised feature that I (in a small way) contributed to is pretty exciting!