You need to provide more detail. Are using a online email service or an email app? It works for me from my Hotmail account . see screenshot Did you try with Firefox in Safe Mode? https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode#w_how-to-start-firefox-in-4troubleshoot-modesf5safe-modesf

A lot of email sites use their own previewer instead of handing the PDF directly over to Firefox. A similar message was mentioned in this AT&T mail support thread where they suggested clearing cache and cookies and, if that didn't fix it, contact Yahoo: https://forums.att.com/conversations/att-mail-features/opening-word-and-excel-attachments-to-inbox-emails/64f9c4d0f78b88060a46da72 A thread in the Sky community says it mysteriously started working for some people a few days ago, so maybe Yahoo rolled out an update, or whatever other software was causing the problem rolled out an update. https://helpforum.sky.com/t5/Email/Email-preview-of-attachments-are-having-trouble-opening-files/td-p/4402132/page/2 Are you still seeing this issue?