Problem install themes and extensions

Helped a friend, who uses Macintosh OSX, to install Firefox 1.0. Then we 
tried to install themes and extensions, it seemed to download them nicely 
and showed a dialogue box where we could accept installation. But then the 
themes did not turn up in the list of available themes, nor did the 
extensions get installed.
I use Firefox on a PC, and haven't had this problem. Any suggestions?

Johan