I love Firefox, it makes me happy. Every once in a while I come up with an idea of something that would make my browsing life easier and most of the time, some brilliant person has figured out some way to make Firefox do it.
Ever click on an email link only to have your email opened in Window's mail? If so, this trick is for you. Let me direct you to this cool lifehacker article:
Once you set everything up, click on a link and then set gmail as your default mailto program. Just remember to "middle click" these mailto links because they will open in the same window as the link.