"Customization." It retains the older, Firefox ~25-ish UI. They've since upgraded the backend to Firefox 52, which is still Really Old. And they've got a lot of culty behavior around their community, if that's your bag.

If you absolutely must have an XUL-based Firefox derivative: 1) you don't, 2) stop, and 3) don't use this one if you don't listen to #1 or #2.