Why do you want to override the automatic settings?

The automatic setting should be optimal, so no need to adjust that.

• Bug 559942 – Heuristic to pick default HTTP cache size

I would like Firefox to be faster by having saved some cache, but not too much cache because I am affraid it could slow Firefox or his opening or it could maybe take too much memory on hard disc. For example in IE9 they tell you that optimum is 50-250MB. I am not IT expert, just a user. Thank you in advance.

You can see the current cache usage and (grayed) maximum via Tools > Options > Advanced > Network

So 1024MB. You suggest me not to override automatic management? Cache will not take more then 1024?