* 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).