Go to the Firefox 3 Bar Menu --> Help ? --> Troubleshooting Information Page and take a look in the Accessibility section if accessibility is set to "true" there. if yes, go to the Firefox 3 Bar Menu --> Options --> Privacy & Security panel and under Permissions check the setting to Prevent Accessibility Services from accessing your browser. Restart Firefox

Please let us know if this solved your issue or if need further assistance.

It was set to "false" already. Thank you.

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Appearance
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

I restarted Firefox in Safe Mode. This seemed to do the trick, so I started it with the extensions on in order to turn them off one by one. The weird thing is that the problem has not returned. I will continue to monitor it. If the problem recurs, I will try again. Thank you.

It seems to be the extension Ghostery causing the problem. So now when I want the PDF open I will disable Ghostery.

I do not have Ghostery. I have Privacy Badger add-on but it is set to disabled on *.office.com domains. Nevertheless when I use outlook.office.com webmail client (seeing that PB add-on icon is greyed-out) I cannot preview any PDF attachment - there is blank 'attachmentlightbox' frame in Firefox Quantum 57 ...