Although you can go and pick up a ready-to-go version of Firefox for Android in Google Play, that particular Firefox version was never made specifically for Android. That particular version of Firefox was made using an interface language called XUL developed by Mozilla.
Using XUL allows Firefox to work and display itself with a similar UI for cross platform use. This means that the current working Firefox UI isn’t taking full advantage of Android.
Last year Mozilla said that they would be working on an Android specific UI overhaul and started to push nightly builds since November for supportive testing. The result of those nightly builds is Firefox Beta.
If you want to be apart of the building process of what seems to be a very great mobile browser, head on over to Google Play and pick up this app, find those bugs and be sure to have them reported.