This is simply inaccurate. Firefox 10 (via changes that arrived way back at Firefox 7) was dramatically better than Firefox 4-6 and Firefox 15 was a good bit better than Firefox 10, thanks to killing add-on leaks and some other minor but incremental improvements in Firefox 11, 12, 13, and 14.
Or to put it another way, Firefox 7 and Firefox 15 both made major advances in memory usage. More memory and performance optimizations hit in 16 or will in upcoming releases with Incremental Garbage Collection, IonMonkey, and then a Compacting Generational GC.
I realize that unsupported assertions based on anecdotes is the norm around here, but expect to get called when they're the opposite of the truth. For the details, read the last few months worth of posts here: https://blog.mozilla.org/nnethercote/