* Firefox Sync. As I use Firefox on my desktop, it's nice to keep them synched.
* Addons. I know this might sound stupid on a phone, but having lastpass autofill your passwords on-the-go is awesome.
* Interface. I really like the Firefox Mobile UI, but I hate the Chrome (Tablet, I browse on a Nexus 7) Mobile UI. Chrome get really bad with 3 or 4 tabs, making it really hard to switch tabs, and really easy to close tabs by mistake.
* Smoothness. I find that Firefox is way better/faster than Chrome on my Nexus 7 (This might be because I run Nightly, but I've compared it to Chrome Beta too). Especially, try to open 2 or 3 pages with a video, Chrome just become unusable for 5/10 seconds for me.
* Memory usage. I have a 2012 Nexus 7, and it has "only" 1gb of ram. It seems that if I open more than 5 tabs, Chrome just love dropping old tabs them out of memory. I've yet to have Firefox drop a tab, even with 10 or more tabs open.
(It does drop them if I leave it in background and open something memory-intensive, like a game, but that's expected).
I can second every point @Spittie wrote. I love Firefox for Android.
To be fair to Android Chrome, it also does Sync if you login with your Google Account. It doesn't handle passwords very well though. I also like how Chrome handles clicks on links - so if the link is too small, or if you're hitting two links at the same time, then it shows you a magnified view of the area you've hit, a feature that's very useful on a mobile phone (not so much on a tablet).
But Firefox did improve by leaps and bounds and has unique features that no other mobile browser currently has, like Addons (AdBlock FTW).
Are you always signed into LastPass on your phone?
I've got a (simple) screen lock password + full encryption, but I'm still a bit paranoid to keep LastPass signed in.
Instead, I have to type my 20+ character passphrase each time I have to use LastPass on my phone, which is frustrating, but not as frustrating as losing all my accounts together with my phone.
I've recently seen more and more ads on pages in Chrome. Just a few days ago, I got fed up and switched to Firefox (with Ad Block Plus installed), and boy am I glad that I did. There are a few places where I feel it's a little slower, and it occasionally frustrates me with links that are close together as it doesn't have the zoom that Chrome does when you hit a link that's sufficiently close to another link as to be ambiguous. But the lack of ads more than makes up for those shortcomings. I've come to realize that I just cannot live without ad blocking any more.
And yes, I do realize that advertising pays for many of the services that I use. It's just that the advertising arms race always makes it more and more intrusive until it's unbearable; I just cannot put up with them. Those services that I value and which offer a subscription plan instead of ads I subscribe to (like LWN). Sadly there's not a lot of content that you can actually choose to pay for rather than receiving via ads, so I just use an ad blocker and move on.
Sync is one good reason if you're already using desktop Firefox.
I had problems using Firefox on a stock Android Nexus 4 (bugs with selecting text and freezing when scrolling) but it's been a flawless experience after switching to Cyanogen. I'm at a loss to explain the difference.
Performance depends on the hardware, it seems. On my dual-core+1GB RAM tablet (a Nook HD), Firefox is visibly faster than Chrome (to the point where the latter is almost unusable). On my N4, they're about the same.
I tried it, found it slower and less responsive than Chrome. But as this is a completely biased feeling, I'd love to know more.