Look - as I said in my other comment, the issue isn't that it uses a tonne of memory. The stock browser does so too. The issue is that it is using that memory in such a way that it is evicted too late. The app is multi-process, but they're putting background services in the wrong process - they run in the main process that you'd expect to be heavy on memory usage. So instead of the memory-heavy UI stuff being evicted shortly after the UI goes away, it is evicted after all other background (multitasking) processes - including the launcher - have also gone away.
Note that my screenshot shows the active (i.e., not background processes that can be thrown away at any time) section of the running processes screen.
I'm wondering how much firefox nightly uses on your phone