It is not a fix since it is not long-term reliable.
I did some test from a liveUSB : https://forum.ubuntu-fr.org/viewtopic.php?pid=21895919#p21895919 and upgrading FF may delete some useful folder regarding icons.
So here is a launchpad bug https://bugs.launchpad.net/ubuntu/+source/firefox/+bug/1760144 please mark yourself as affected 