Firefox 4 b12: no cache dir setting

I thought about moving the firefox cache directory to /tmp, as I am moving /tmp to the ram, since I just got an SSD, and move off this write-hungry feature.

However, with Firefox 4 beta 12 (shipping with 11.4 x86_64), when I get in about:config, there is just no option browser.cache.disk.parent_directory, which you would use to configure this.

I searched a bit and it seems that this is still intended to be on Firefox in v.4

Humm… I might as well downgrade to 3.x for the moment. I see there is an RC available of v.4, but no package of it ready in repos.

By the way, any thoughts on putting the firefox cache on ram? I know it will flush it at reboot! But the alternatives are:
-leave it on ssd (wear it)
-put it on hdd (so slow)
-disable cache

SOLVED IT

Solution:
about:config
If you do not see the string browser.cache.disk.parent_directory, then right click in anywhere on that screen > New > string > type browser.cache.disk.parent_directory > set directory as what you want (in my case /tmp)

Discussion:
I downgraded without problem to firefox 3.x on openSUSE 11.4. It was not there! I tried Windows Firefox through wine. Again, It was not there!

Source: (see step 2)
Firefox cache location | InfoHole

What wasn’t there? The change in cache directory?

When you get in about:config, and you type “browser.cache.disk.parent_directory”, you get no result. Well, I had no result. So to configure this setting, you must add the entry for this setting to exist.