Access to Firefox Extensions Chrome Directory?

I just installed the Firefox extension, Greasemonkey, which lets you 
install Greasemonkey scripts that can add features to web pages you 
visit. So then I went to install a Greasemonkey script, and I got the 
error message:

> Failed to create directory. You probably don't have appropriate permissions 
> (write access to mozilla/chrome directory). Error code: -215.

I used Spotlight to search for a Chrome directory, and quite a few 
directories with that name turned up that were associated with Firefox. 
They all seem to have permissions saying "You can Read & Write."

How can I fix this? Thanks in advance to all for any info.