On the first machine, after clicking on a PDF file, Firefox pops up the usual "What should Firefox do with this file?" dialog, with "Open with: Document Viewer (default)" setting. Clicking OK downloads the PDF file, then opens it inside Firefox's window.
On another machine, after doing the same thing, Firefox downloads the PDF file, the croaks with this error dialog:
"/tmp/<filename> could not be opened, because the associated helper application does not exist. Change the association in your preferences".
I can manually open /tmp, and launch the PDF file, which opens in evince normally. Both firefoxes have the same "PDF document" setting in Preferences/Applications, and both machines have identical /etc/mozpluggerrc
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