That's good to know, I don't know how well it would work though I feel like I enabled something about closing background tabs to reduce memory load (it might have been what you said, it might have been something else I don't really remember) and it helped a little bit but it still ended up chewing up a lot of memory.
Setting the limit though did help immediately. And stop the overconsumption problems, occasionally a couple of tabs crash here and there but it doesn't freeze or worse cause other apps to slow down and freeze. Which did happen before.
No, it just limits the amount of RAM that Firefox (or whatever other application you launch with these parameters) will see.
A few Firefox tabs may crash occasionally as a side effect. And obviously a Firefox eats up all of the 8GB it's allocated it may crash itself though usually it doesn't and tabs will crash before the browser crashes.