Probably the biggest change to Firefox is what's called Panorama. It lets you visually see a preview of all of your open tabs and group them together. It's very, very useful for people like me that always has a million tabs open and struggle to find the one I want. The button is at the top right.
Click and your tabs will look like this:
All of your tabs are grouped together and you can see a preview of them. If you have, say, 30 tabs open, this is a great way to switch between them.
But, that's not the greatest part of this. You can group similar tabs together to make things so much easier. Let me show you a video of me demonstrating.
I am currently working on a research paper for school (and probably should be working on that now lol) on the Japanese religion Shinto, so I have a bunch of tabs open to get information from. In the video I grouped all of the tabs that deal with Shinto together. That way, they can become separate from all of my other open tabs (and thus I don't have distractions like checking my email every five minutes nor do I have to worry about finding Shinto related tabs among 30 other unrelated tabs).
There are tons of other things you can do with Panorama but I'll leave that to you to figure out!