Hello, Try make the change in safe mode, then try set the PDF viewer as default: *[https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode Firefox in safe mode] See this help articles: *[https://support.mozilla.org/en-US/kb/how-to-fix-preferences-wont-save Preferences won’t save] *[http://kb.mozillazine.org/Preferences_not_saved]

thanks, for the reply, but it still wont open using the default firefox pdf viewer, do you know any ways how to re-enable it.

You can change the action for Portable Document Format (PDF) tom "Preview in Firefox" in "Firefox > Options/Preferences > Applications". *https://support.mozilla.org/kb/change-firefox-behavior-when-open-file Set the pdfjs.disabled pref to false on the <b>about:config</b> page to enable the build-in PDF viewer. See also: *https://support.mozilla.org/kb/view-pdf-files-firefox-without-downloading-them ---- The latest stable version of the built-in PDF viewer is available as an extension and this version can possibly perform better than the version shipped with the current Firefox release. The latest stable version of the PDF Viewer (pdfjs) extension: *PDF Viewer: https://addons.mozilla.org/firefox/addon/pdfjs/ Latest development version of the PDF Viewer (pdfjs) extension: *http://mozilla.github.io/pdf.js/extensions/firefox/pdf.js.xpi

thanks for the help.