In this case, FF 65.0.1.
Yeah, I guessed. Firefox introduced webp support in 65.0, but didn't
quite get the config right (it will be fixed in 66.0).
If you are happy making config changes, I can give a workaround. It simply changes a preference to the value it will have in 66.0
In the address bar type
about:config; press Enter; accept any warning to be careful or similar
In the search bar at the top type
http.acc; wait while the list is filtered
Go to the preference
network.http.accept.default (probably the last one)
Double-click the pref and edit the value to remove
exactly the characters
image/webp,Its value should now read
text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8In the search bar at the top type
image.webp.enabled; wait while the list is filtered
Double-click the pref to set its value to
falseClose the about:config tab and restart the browser. Job done.
Edit: add missing step