One month ago I was experiencing memory leaks with Firefox.
I did a fast check with "top" command and seen that when Firefox was running it started to overcharge Xorg, making it to use more and more CPU and Memory resources, near 95%!!!!!!!!
So I've decided to use Opera. That was a nice experience to use that super-fast browser. The problem is that Firefox is a key tool for my working day (I'm web developer).
Today I've launched Opera from the command line, and noticed this warning:
"opera: X Shared memory extension is not available. ZPixmap not supported"

Then I found the following with google:
xf86-video-intel 2.2.0 uses EXA by default, which is complete crap right
now (I don't know why they turned it on by default, guess it's to get
better testing and more bugreports). Add this to the section of your
videocard to switch back to XAA:

Option "AccelMethod" "XAA"

and you shouldn't have trouble anymore. Note that there's a new
xf86-video-intel driver available upstream, which is a 2.2.1 prerelease
with quite some bugfixes. I'll get to that one shortly.
read complete thread

I added that line to my xorg.conf:
Option "AccelMethod" "XAA"

Restarted my computer, launched Firefox and monitored CPU and Memory Usage with the help of "top" command.... And woww, firefox works really fast, scrolling is smoothness, CPU usage is between 15 and 50% and no more memory leaks.

Blessings!