Can't play runescape with firefox 3.0 on ubuntu 8.04

I went to play runescape which is now all HD, and this error came up Error_loader_nocache - Unable to create cache directory.

Runescape was unable to find a suitable place to store its temporary files. There are 4 possible solutions:
Log-in as an administrator
Log-in to your computer as an 'administator' user, and then try loading RuneScape again. This should give it sufficient access to create its temporary cache.

Create directory manually
Create a new directory called c:\rscache or /rscache. If possible, set that directory to have full read+write permissions so that all users can write to it. Runescape should then detect that directory and use it for its files.

Try running in the windows Client
Try downloading the RuneScape Windows Client by clicking here, and use that to play the game. The windows client launches RuneScape directly from your desktop. (Requires Internet Explorer 6 or above).

Unsigned Applet
As a last resort you can tell RuneScape not to store any files on the harddisk. This will cause the game to load slower, and also won't fix the 'high detail' version of the game.

    * Go to the RuneScape home page.
    * In the 'Play now' menu, choose 'Java Options'.
    * Select 'Unsigned applet' and finally click 'Save Settings'.

I tried everything except making a cache dictionary for it, manually. The reason for this is I just don't know how. Can someone help me? How do I manually make a cache using firefox 3 on ubuntu 8.04? Please help me, thanks.