Whether your code will run well on a phone depends on which browser(s) you test in. The entire apps stack is also being implemented in Firefox for Android (it was Firefox Aurora as of September):
In other words, Mozilla is already dealing with plenty of devices today. I'd expect that, in the majority of cases, if an app works well in Firefox for Android, it will work well on Firefox OS and vice-versa. After all, the whole point is to enable apps that are portable to any device/browser combination that implements the relevant APIs.
https://developer.mozilla.org/en-US/docs/Apps/Apps_for_Andro...
In other words, Mozilla is already dealing with plenty of devices today. I'd expect that, in the majority of cases, if an app works well in Firefox for Android, it will work well on Firefox OS and vice-versa. After all, the whole point is to enable apps that are portable to any device/browser combination that implements the relevant APIs.