I have been using my Nexus 5 all day and find that Firefox for Android is laggy/slow. I have cleared my Private Data and that helped for a bit. I really notice the slowness when opening links from Twitter for Android and when at https://twitter.com and pulling to refresh is funny to watch in slow motion. I have none of these issues with Chrome or Chrome Beta. For the rest of the night I will be using my Moto X and will see how this goes