To get a preview, go to Tools/Options/Applications. Scan down to download/attachment and on the right it probably says "save file". Click on it and change it to "always ask" then click OK. That should fix it.

If it already says "always ask" and you are not getting the preview, you got a different problem that I don't know the answer to.

I was running FF 7.0.1, & having the Same problem - I Solved it... I loaded 8.0 (Beta 2), then 3.6.23... problem persisted. I use "PDF-XChange Viewer"... I Uninstalled it, then Installed the newest version (2.5.199.0). That Solved the problem in all 3 of the above versions of FF. I believe it has something to do with the "Viewer" Plug-In being contained within the install file.

Mozilla Help refers to this procedure, but they refer to "Acrobat" specifically. Evidently it works with more than just Acrobat. Hope this helps someone else, so they need not spend as much time on it as I did. MAKE a nice day... pjdc3