Those are mine:

* 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).

Firefox for Android supports add-ons like Adblock Plus: https://addons.mozilla.org/en-US/android/addon/adblock-plus/

Firefox for Android also runs on Android 2.2 (Froyo) and later. Chrome only supports Android 4.0 (ICS) and later.

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.

I use Firefox on Android for its proxy settings, so I can SSH tunnel out of China.

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.

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