Make sure you are not blocking content.

Start Firefox in Safe Mode {web link}

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop

Hi Thanks for your reply The attachment was a jpeg which was downloaded but not opened when clicked on In safe mode same thing- the attachment is downloaded but not opened.

Hi mike.lawley,

Google says that if you are facing this problem, then you can try these 3 solutions.

• On your computer, check that you're using a supported browser.
• Try turning off extensions you have on your browser one at a time.
• Clear your browser's cache and cookies.

So, for the first Solution, Firefox is well Suppoted. There is no any chance of that. For second Solution, you have tried safe mode and found same result. So, For Third Solution, You can try clearing Cookies and Sites Data on Your Browser :-

• Click the menu button. and select Options. Preferences.
• Select the Privacy & Security panel and go to the Cookies and Site Data section.
• Click the Clear Data… button. The Clear Data dialog will appear. ...
• Click Clear.

If it doesn't solved your problem, then feel free to ask in Reply Section. Thanks for raising your question in Mozilla Support Forum.