I'm not a Firefox user, but considering that 95% of the time about half my RAM is going unused (well, Windows uses it for pre-caching random stuff it thinks I might need), I don't mind high memory usage as long as I get a benefit for it.
If it's just inefficient, that's a different story.
The thing is HD caching is extremely elastic. Memory consumption by a browser, less so (maybe even nearly not at all elastic).
So what you might think of a benefit one day might be a drawback the next if you want to open twice the same amount of tabs, or run a big build in the background.
If it's just inefficient, that's a different story.