I want to open PDF files in Firefox's default PDF viewer in their own tab, without downloading them. I am opening them from attachments in Gmail. When I try to open them, one of several things will happen: 1) They will open a new tab, and prompt me to select how to open. Selecting to open them in Firefox simply causes a new tab to open with the same prompt. Selecting "do this automatically" results in an infinite new-tab explosion. Downloading them and opening them in Firefox has the same effect. 2) After the first error occurs, they will open a new tab, but with an alert that there has been a Gmail server error. Temporary Error (500) 3) Sometimes they start to just download and not open any new tabs. Sometimes I have gotten the case in (1) to work after 2 new tabs, but otherwise nothing. It is definitely Firefox, because it works in other browsers (I'm posting here because I would just like to get Firefox working), and they open fine in the Adobe application. I have tried everything described on the Firefox troubleshooting pages: I have opened the browser in safe mode. I have restarted my computer, Firefox, etc. I have uninstalled and reinstalled Firefox. I have checked the settings in the Applications tab in Options. I have repaired Adobe Reader's installation. I have deleted mimeTypes.rdf so Firefox could generate a new one. Everything is up-to-date. What else is there for me to do? Let me know exactly what info you need, and I'll do my best to provide it.