The feature I use most is Firefox Sync and the ability to not only send browser tabs to instances of Firefox running on other machines but also to look at and l

The feature I use most is Firefox Sync and the ability to not only send browser tabs to instances of Firefox running on other machines but also to look at and load tabs from another machine, even when it is offline. Routinely I'll be reading something on my laptop in the evening and want to reference it from my Dev VM the next day. With Sync I can just pull up the list of open tabs on my laptop that's asleep and retrieve the URL.