I recently updated to Ubuntu 10.4 and Firefox 3.6.13. Was on previous Ubuntu LTS and not sure what Firefox. When I select a PDF file from a web site, it will download but then does not open. I have played with the settings and if I select: Preferences -> Applications -> PDF file -> use acroread; the file will download and nothing else. If I select: Preferences -> Applications -> PDF file -> Use Default Viewer; the file will download and open in a new window in an application call "Document Viewer". I prefer this behavior, but with Adobe Reader, this is how it has worked for the past 6+ years.