As good as Firefox is, and as good as the Mozilla folks are, This is one dragon they have never slayed. Firefox has had a memory leak since inception. They have never fixed it. One tab open or many, doesn't matter. Leak, leak, leak. I have posted a ton about this in other forums.
Take an ordinary machine...my wife's...a Celeron running XP Pro with 512 Gb of RAM. OPen Firefox to a single page...Yahoo. Wait. It eats some RAM. Wait some more and it eats more. Eventually, it takes all the RAM and crashes itself or the machine or both. I've watched this happen. Granted, it takes a couple of days, but it will act like a flesh eating bacteria and consume everything including itself.
To me, that's a BIG problem. Working in software myself I know memory management can be a bear. But this is bad...and has been forever.