As of August 16th, or shortly after, Firefox 6 will be released on the stable channel, 7 will move into the Beta Channel and 8 will move into the Aurora channel. If you can't wait for 7 to be released in the stable channel then download it now: http
/www.mozilla.com/en-US/firefox/channel/
Remember, just because it's not in the stable channel, doesn't mean it isn't available.
One thing that you have to remember with the memory consumption on Firefox is that these improvements affect Firefox, they do NOT affect add-ons.
So, if you're sat there using Firefox 4 with 20 different add-ons installed, moaning about the amount of memory that is used, try installing Firefox Aurora 7 today, with no add-ons and then seeing how much faster it is. It will blow you away.