So just had an issue with favicons not showing in Firefox v70.0.1.
Now this will probably only be an issue for a very few people and most likely only if you have modified the about:config settings in FF
This issue for me was the setting privacy.resistfingerprinting was set to true on some sites this will block favicons.
The other issue I noticed and what lead me to find out what was causing the issue was using dropzone.js on a site I was creating the uploaded images had a blank preview. The previews are base64 encoded and were being blocked by this.
Like I said most likely wont be an issue for most but wanted to post this somewhere so it may help others that may be facing a similar issue as I did.
See more here: https://wiki.mozilla.org/Security/Fingerprinting
And Here: https://bugzilla.mozilla.org/show_bug.cgi?id=1452391