What is the best way to support Mozilla/Firefox? I'm happy to sign up for this if it is the best way to contribute to the longevity of the product I use more than anything else.
The Mozilla Foundation exists to support internet freedom. Firefox is a means to that end, but if they fired every Firefox developer and replaced them with a skin on top of Chrome that wouldn't be against Mozilla's goals.
Which means: if you want to support Mozilla you should sign up for this and some of that money will trickle down to Firefox. If you want to support Firefox then you should convince people to use it more.
It's probably still Google and I generally encourage people to get weaned off the Google product teat as much as possible, but that search engine integration is how Moz makes most of their money from what I understand.