You must create Browser.cache.disk.parent directory , see in the link how to do that, but be careful.

thank you

it works but still temporary flash files (e.g. from youtube) are stored in the same location - C:\Users\user_name\AppData\Local\Temp. Is any solution for that?

You need to change the location of the temp files in Windows in the control Panel if you want to use another location for temp files.

You can change the %TEMP% and %TMP% environment variables and make them point to a temp folder that you want to use.

• Control Panel > System > Advanced > Environment variables

yes i know that, but I don't wont change the environment variables in my account. I just wont to change only firefox settings. It can be done?

No, Firefox uses the OS temp file location for storing temporary files that are passed to external program or during the initial (background) phase of a download before you've specified the location.