If you mean it showed the out of memory dialog, that wouldn't be caused by an app using 3GB. The dialog shows up at ~48GB swap space used on an 8GB Mac, or when you're out of disk space and can't write a swap file.
It’s a losing battle me trying to tell my wife to close her Firefox tabs, haha, but yes, Firefox does use a lot of ram when you have 500 tabs. Maybe I’ll get her a 64GB MacBook Pro for the premium web browsing experience she so desires!
I do it myself and I'm sure a lot of people on HN do too. But I've tried to embrace the "zen" of closing all tabs lately and it's been nice. If I really want to find something later I can search my history or, like you said, just bookmark it.