By moving the Firefox network cache from your hard disk to the RAM, you diminish the amount of disk writes. This will probably make your Firefox noticeably faster as well. The price you pay is small: it'll only “cost” you 200 MB of your RAM.

Note: don't do this when your computer has less than 2 GB of RAM! Because with very little RAM, even 200 MB can't be missed.

Proceed like this:

a. Type in the URL bar of Firefox:

about:config

Press Enter.

b. Now you're being presented with a warning. Ignore it and click on the button “I accept the risk!”.

c. Copy/paste the following into the filter bar (search bar):

browser.cache.disk.enable

Toggle its value to false by double-clicking it: this will disable “cache to disk” entirely.

d. Now you're going to make sure that “cache to RAM” is enabled. Copy/paste the following into the filter bar (search bar):

browser.cache.memory.enable

This should already be set to true; if not, toggle it to true by double-clicking it.

e. Then you're going to determine how much memory can be used as RAM cache. Copy/paste this into the filter bar (search bar):

browser.cache.memory.capacity

You'll probably won't find an existing instance. If (as expected) nothing was found, create it like this:

Right-click on the blank area in Firefox, then select New - Integer. Make this the preference name:

browser.cache.memory.capacity

Enter the integer value in KB; I advise 204800 (which equals 200 MB). That's usually more than enough.

f. Close Firefox and launch it again. You're done! Check it like this:

Type in the URL bar:

about:cache

Press Enter.

By the way: you'll then also see a mention of an “appcache” which is still present on the disk, but there's no need to move that (much less frequently used) cache to the RAM as well.

Note: This is a user preference. Repeat this hack in each user account.