It's a shame this is happening at Mozilla. I was also quite shocked by their recent article to "safely return to shared memory between tabs", which I immediately thought was a bad idea.
Lo and behold, a mere week after that update, I found that Firefox was eating up my entire memory and making the PC perform poorly.
Shared memory is what made the old Firefox hang so much - I can't believe they thought it was a good idea. I've always believed Chrome's per process tab idea was brilliant, even if it uses a little more memory on average.
I still prefer the security benefits and stability of that architecture over Firefox randomly hanging and peaking in memory use due to memory leaks even if it can use slightly less memory on average.