If images are missing then check that you do not block images from some domains. You can tap the Alt key or press F10 to show the Menu Bar Check the permissions for the domain in the currently selected tab in "Tools -> Page Info -> Permissions".

Check "Tools -> Page Info -> Media" for blocked images

• select the first image and use the cursor Down key to scroll through the list
• if an image in the list is grayed and "Block Images from..." has a checkmark then remove this checkmark to unblock images from this domain

Check permissions.default.image on the about:config page to make sure it is default (1).

There is Tracking Protection that can block content, so check the location/address bar for special icons.

• https://support.mozilla.org/kb/control-center-site-privacy-and-security-firefox

If you use extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) that can block content (e.g. Adblock Plus, NoScript, Flash Block, Ghostery) then make sure such extensions do not block content. There is security software that can interfere with loading content, so check that as well.

See also:

• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes
• http://kb.mozillazine.org/Images_or_animations_do_not_load
• http://kb.mozillazine.org/Websites_look_wrong

I solved the issue by holding the shift key and clicking on reload page icon, and suddenly all images on the gallery showed up.

Can you tell me why this happened?

Thank you

This means that the images in the cache were corrupted or possibly there has been a problem with the server that stores to connection details to that server.

You can reload web page(s) and bypass the cache to refresh possibly outdated or corrupted files.

• Hold down the Shift key and left-click the Reload button
• Press "Ctrl + F5" or press "Ctrl + Shift + R" (Windows,Linux)
• Press "Command + Shift + R" (Mac)

Thank you!