Firefox is fast and being a different engine helps set itself apart from almost every other browser out there. It also has a lot of customization, it's got a lot of different things you can do with extensions, like vertical/tree style tabs you can see a few here, there's also gesturefy which lets you navigate the browser with mouse gestures. Like @Pirate said there's also container tabs which are now in the base browser, but extensions can use it to do more like facebook containers which isolates facebook related tabs and trackers into a facebook container seperate from everything else, or temporary containers which is like a mini incognito tab, or google containers among others. There's a lot more but that's what I can think of off the top of my head.
Lastly, since most browsers use the WebExtension API extensions are mostly cross compatible which makes it possible to run chrome exclusive extensions on firefox based browsers as well, you will probably have to compile it yourself or get a converter but waterfox even goes so far as to let you use the chrome web store if you want, though obviously it's better to use extensions from firefox extensions when possible because it has better moderation and the extensions are guaranteed to work.
Firefox also has a thing called userChrome (which is called that because of chrome=new look, it came out before the chrome browser lol) which lets you basically change the whole look of the browser, youcanseesomeexampleshere. Those are a few examples but if you want more you can check out /r/firefoxcss. There's a redesign for firefox coming soon so they might need some reworking after that but I'm sure they're aware and working on them.
Outside of customization firefox has other nice things going for it. The firefox accounts system isn't locked to firefox itself. I can use waterfox at home and firefox on my work computer and fennec on my phone and they would all sync up with the "tabs from other devices" button if I'm using the same firefox account. There's also firefox profiles which are basically completely different firefox instances and while the interface for changing them is normally pretty rough there's an extension (and a little piece of software) that makes it just as easy as chrome.
Firefox is fast and being a different engine helps set itself apart from almost every other browser out there. It also has a lot of customization, it's got a lot of different things you can do with extensions, like vertical/tree style tabs you can see a few here, there's also gesturefy which lets you navigate the browser with mouse gestures. Like @Pirate said there's also container tabs which are now in the base browser, but extensions can use it to do more like facebook containers which isolates facebook related tabs and trackers into a facebook container seperate from everything else, or temporary containers which is like a mini incognito tab, or google containers among others. There's a lot more but that's what I can think of off the top of my head. Lastly, since most browsers use the WebExtension API extensions are mostly cross compatible which makes it possible to run chrome exclusive extensions on firefox based browsers as well, you will probably have to compile it yourself or get a converter but waterfox even goes so far as to let you use the chrome web store if you want, though obviously it's better to use extensions from firefox extensions when possible because it has better moderation and the extensions are guaranteed to work.
Firefox also has a thing called userChrome (which is called that because of chrome=new look, it came out before the chrome browser lol) which lets you basically change the whole look of the browser, you can see some examples here. Those are a few examples but if you want more you can check out /r/firefoxcss. There's a redesign for firefox coming soon so they might need some reworking after that but I'm sure they're aware and working on them.
Outside of customization firefox has other nice things going for it. The firefox accounts system isn't locked to firefox itself. I can use waterfox at home and firefox on my work computer and fennec on my phone and they would all sync up with the "tabs from other devices" button if I'm using the same firefox account. There's also firefox profiles which are basically completely different firefox instances and while the interface for changing them is normally pretty rough there's an extension (and a little piece of software) that makes it just as easy as chrome.