Hmm I hope it is much improved, I'd love to go back to using it as my primary. It's a lovely browser in many ways, just recently I was having issues with it using so much memory and causing my whole desktop to become sluggish so I've been using Chromium.
Something defiantly changed, FF14 on my android was barley usable, FF15 is pretty smooth, and more responsive. The only complaint is that not all text on sites like this forum, is being scaled the same.
FF15 sure fixed some memleaks for me. I use about 15 addons and leaving the browser open for a few days, with 30+ tabs open, would send RAM usage to 1.5Gb+ in FF14. Now it seems stable below 400 Mb under the same load (I switched to the beta a month ago and didn't look back).
About chrome vs firefox, I once saw some benchmarks (tom's hardware, I think) testing mem usage under different conditions, where firefox had much higher usage when started (fresh - one tab). However, going to several tabs caused chrome to increase dramatically, while firefox only increased slightly. So if you don't use tabbed browsing that much, chrome could be the better choice - firefox, when using many tabs. Though features and addons is what kept me with firefox through a few shaky releases..
Well, I saw that FF15 was released, so i upgraded firefox-nightly from 17.0a1 to 18.0a1 ...
I haven't used any version of firefox, but firefox-nightly for a long time now (FF6 was the last 'official' release that i used). One of the nice things with newer versions of FF is that plugins tend to not break on upgrade for me, anymore. Where as they used to break, quite consistently. RAM usage has been noticeably lower for a few releases and i like how they cleaned up the downloads part of the UI.
It seems like every release they say memory usage is improved but I don't see much noticeable change. It's starting to feel meaningless like how every Ubuntu release is said to be faster, smoother, have quicker boot time, feel "snappier".
i've never noticed a memory issue. this is on a netbook with 2GB, with typically 20-30 tabs in 3-5 windows. I'd be very happy though if they reduced CPU usage when firefox is 'idle'. it tends to sit at a constant 10-20% even when i am not interacting with it (this is without flash installed, noscript blocking some but not all scripts, gifs set to play once).