Can't paste file name to web pages saved as PDF...

Whenever I try to save a webpage as a PDF file, I have to manually type in the name of the file. For some reason, Firefox will not let me paste the file name (and some of them are very long!).

I am running Firefox 3.5.4 on Mac OS 10.4.10.

Is there any fix for this?

Thanks a bunch for any insights on this problem!

Gwen