My understanding is that the RAM per tab metric drops as you have more tabs. Firefox is more willing to cache stuff when you have only 4 tabs open, so that's not a great metric. Plus you have to consider the base RAM use of the browser itself. 1.3 GB isn't all that much on a modern system with 8 GB available, for example; the problem is more that Firefox doesn't handle 20+ tabs very well especially when you've got Twitter, Google Docs, Slack, etc open.