Temp files

Each time I open a PDF file using Firefox it puts a copy of the file on my desktop. Somewhat aggrevating.

I can use the preferences in Firefox to store the files in a desktop folder then manually empty the folder or....

I could put the files into Library/Caches/TemporyItems.... If I put them there will OSX automatically clean them up for me or is it just another manual workaround? If its just another manual procedure I'll use the desktop folder....

Thanks