Did you check this in the Inspector?

• https://developer.mozilla.org/Tools/Page_Inspector

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 link 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.

Make sure that you do not block (third-party) images, the permissions.default.image pref on the about:config page should be 1.

There are extensions like Adblock Plus (Firefox menu button/Tools > Add-ons > Extensions) and Anti-Virus software that can block images and other content.

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 assume you cleared the cache to rule out issue with damaged or obsolete script files?

You could temporarily rename prefs.js to prefs_old.js to bypass those settings. If it makes a difference, you'd know it was something in there.

Note that this can also be a problem with exceptions (Site Preferences).

If the above doesn't help then you can try to use "Forget About This Site".

You can remove all data stored in Firefox from a specific domain via "Forget About This Site" in the right-click context menu of an history entry ("History > Show All History" or "View > Sidebar > History") or via the about:permissions page.

Using "Forget About This Site" will remove all data stored in Firefox from that domain like bookmarks, cookies, passwords, cache, history, and exceptions, so be cautious. If you have a password or other data from that domain that you do not want to lose then make sure to backup this data or make a note.

You can't recover from this 'forget' unless you have a backup of the involved files.

If you revisit a 'forgotten' website then data from that website will be saved once again.

There was a parameter in about:config that broke site functionality: dom.w3c_touch_events.enabled was set 1 (instead of default 0). The moment I reset it everything starts working as expected.

UPD: It's smarter to set preference to 2 ('auto-detect', as MDN teaches us). Toggling 'simulate touch events' button in 'Responsive design view' mode toggles pref. value between 0 and 1, but never 2.