Hello

I have Ubuntu(Fiesty 7.04) and firefox version 3.0.
Whever i try to open a file (like a pdf) from internet, it displays me the following message and
The application you chose ("(null)") could not be found. Check the file name or choose another application.

than suggest
"try saving the file and than open it"
.


I do not have root access and just the user on a system, how can i set it right.

-Thank you