Hi, you can set up some stuff here if it is listed, to tell it what to do or what not to do :

• https://support.mozilla.org/en-US/kb/applications-panel-set-how-firefox-handles-files
• http://www.idownloadblog.com/2015/05/15/how-to-change-default-image-viewer-mac/

There are other ways if the above does not work.

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

Pkshadow, Thanks for the links but I've already done the suggestions in both of them.

I did notice today that it happened specifically when i uploaded photos to Google Photos. Not sure if that makes a difference.

Thanks, GES

I think the viewer could be a O/S related file assciation. Did you see if the Mac O/S perferences changed.

I don't understand what you mean by "Mac O/S perferences changed"? What exactly should I check?

See also: How to change the default application for a file type | Macworld:

• http://www.macworld.com/article/1154861/changedefaultapp.html

Cor-el - thanks for the forward, have done that multiple times, still reverts back seemingly at random.

HI, please see your Mac Forum for answers as this is not a Firefox issue but a Mac issue to be figured out with Mac Support from a Community Forum.

Why is it a Mac issue when the default viewer doesn't switch to any other program on my Mac but Firefox?

Hi, it is a Mac problem as Firefox should not be the default viewer and your OS is in charge of such things when set up properly.

Please see your Mac Forum of Your choice to find a answer for a Mac Issue.

Thank You for Contacting Mozilla Volunteer Support.

I have posted this thread over on the Apple Support Communities forum. I hope this thread will be left open for me to return to in case no one is able to offer any workable solution over there, and or for me to cross-post the workable solution here if one is in fact found. Thanks, GES

Try to update to Firefox 61 as this might be fixed in Firefox 61.

• Firefox -> About Firefox

This seems to be a sandbox security issue, so if you still have the problem in Firefox 61 then try to modify this pref on the about:config page and change the value from 3 to 2.

• security.sandbox.content.level = 2
• close and restart Firefox to make the change effective.

If this didn't work then undo/reverse the change and reset the pref via the right-click context menu to the default value.

You can open the about:config page via the location/address bar. You can accept the warning and click "I accept the risk!" to continue.

• http://kb.mozillazine.org/about:config

See also:

• https://wiki.mozilla.org/Security/Sandbox#Content_Levels

Cor-el - Thanks for the additional suggestion. The issue still persisted after upgrading to Firefox 61, but I didn't have a chance to try out your security sandbox idea because the following Apple Support Community trick took care of the issue:

"Another thing that has come to mind involves the com.apple.finder.plist

As before Select the Go To Folder option from the Go menubar item in Finder type in ~/Library/Preferences press Go

scroll down to com.apple.finder.plist move this to your Desktop, then Restart you Mac. Once restarted highlight an image (jpeg) select Preview as the default app and then Change All, now double click another image and see if Preview is chosen as the default. Again Restart and see if this time the change has happened."

If the issue ever comes back again, I will be sure to try the security sandbox suggestion.

Thanks again, GES