hi, i think this problem can arise if you set firefox to be the default pdf-viewing app in your windows settings AND have chosen to open pdfs with the system's default app in the firefox settings - you'll then end up with this endless loop.

in order to resolve this, go to the firefox menu ≡ > options > general panel and in the applications panel set the default action for portable document format (pdf) to preview in firefox.

this solves it  !

preview was the required setting.