See this support article. <br /> https://support.mozilla.org/en-US/kb/fix-printing-problems-firefox#w_reset-firefox-printer-setting

Clear the cache and cookies only from websites that cause problems. "Clear the Cache": *Firefox/Tools > Options > Advanced > Network > Cached Web Content: "Clear Now" "Remove Cookies" from sites causing problems: *Firefox/Tools > Options > Privacy > Cookies: "Show Cookies" If this is a problem with a PDF file then you may have to disable the built-in PDF Viewer. Current Firefox versions have enabled a built-in PDF Viewer that doesn't have all features that other PDF readers like the Adobe Reader have or may not function properly otherwise. See also: *https://support.mozilla.org/kb/view-pdf-files-firefox-without-downloading-them