It could be lots of things. One method to suss out an issue with settings or add-ons is:

New Profile Test

This takes about 3 minutes, plus the time to test your problem site(s).

Inside Firefox, type or paste about:profiles in the address bar and press Enter/Return to load it.

Take a quick glance at the page and make a mental note of which Profile has this notation: This is the profile in use and it cannot be deleted. That is your current default profile.

Click the "Create a New Profile" button, then click Next. Assign a name like Test2022, ignore the option to relocate the profile folder, and click the Finish button.

Firefox will switch your default profile to the new one, so click the Set as Default Profile button for your regular one to avoid an unwanted surprise at your next startup.

Scroll down to Test2022 and click its Launch profile in new browser button.

Firefox should open a new window that looks like a brand new, uncustomized installation. (Your existing Firefox window(s) should not be affected.) Please ignore any tabs enticing you to connect to a Sync account or to activate extensions found on your system to get a clean test.

Do the problem site(s) work any better in the new profile?

If they don't work better, the problem most likely is outside of Firefox, for example, the site, your security software (if it intercepts and filters your browsing), your network, or Mac OS.

If they DO work better, you could compare the Preferences pages between the two windows (your regular profile window and the test profile) and see whether you notice any configuration differences that might be a factor.

When you are done with the experiment, you can close the extra window without affecting your regular Firefox profile. (Test2022 will remain available for future testing.)

Make sure Firefox isn't set to run in permanent Private Browsing mode (Always use Private Browsing mode; Never Remember History).

• https://support.mozilla.org/en-US/kb/private-browsing-use-firefox-without-history
• https://support.mozilla.org/en-US/kb/common-myths-about-private-browsing

• Settings -> Privacy & Security
  Firefox will: "Use custom settings for history"
• remove checkmark: [ ] "Always use Private Browsing mode"

jscher2000 - Thanks. I did try that, but it made no visible difference.

Then, on a page that wasn't loadiing, I saw a button in the frame atthe bottom of the Firefox browser window that led me to this page:

https://support.mozilla.org/en-US/kb/refresh-firefox-reset-add-ons-and-settings?redirectslug=reset-firefox-easily-fix-most-problems&redirectlocale=en-US

That seems to have fixed the problem. But, naturally enough, the image upload problem on WhatsApp persists – a coincidence.

Cor-el,

Thanks. I have 'remember history' set, though.

I spoke too soon. Just as bad as ever, maybe worse.

Are we sure Firefox 78.15.Oesr isn't being obsoleted?

Some sites no longer support Firefox 78 because it is a year out of date now, but I don't know about WhatsApp.

jscher2000 – Thanks for that. FWIW, it started to happen to a lot of sites (that I regularly visit) pretty much overnight, on approximately 24th Sept: whatever day it was, I switched on in the morning and maybe 25% of the sites i visit daily were suddenly 'malfunctioning'

Now, I don't know web architecture or programmimg from an oak tree, but because the effect was (1) widespread and (2) simultaneous, I have to suspect that some widely-used 'module' or whatever it's called that sites use had an upgrade or a change or what-have-you; also that whatever the change was, it relates specifically to the way images are uploaded and downloaded.

The (admittedly circumstantial and imperfect) indicators of that are:

1. It seems to be pages that serve frequently updated graphics (news sites, Google maps - even Google search because it's configured to offer image 'hits').
2. At the same time, as I said, I was no longer to upload photos on my desktop WhatsApp app (though, fwiw, I could see and download images sent to me by others fine, still).
3. Images on some emails would not download either (Mac Mail 9.3–3124)

So it seems to be all about image transfer, simply because three different kinds of app suffering essentially the same failure at the same time is extremely unlikely to be a coincidence.

Also FWIW, the closest thing to this that's ever happened before is that a few months ago, videos stopped loading for me on CNN's site on Firefox – but I tried the same page and the same videos on Safari, on a whim, and that was still working.

I dunno. Do you know whether any part of image processing / exchange is facilitated or governed 'upstream' in Mac OS, rather than in applications that run on the OS? If so, then a likely possibility is that it is my old Mac OS, 10.11.6, that is unable to work with some change in web architecture that we might now suspect.

Thanks again for your thought.

As a Windows person, I don't have much insight into Mac OS.