For the past several days the Gmail icon (envelope with red M) which normally appears in the upper left corner of the window while using Gmail has refused to render (ie, just a greyed out rectangle with a broken icon sign).
Not having any problems with alternative platforms and browsers (eg, Windows 10 + Chrome/IE), it would seem that Firefox Quantum (+ El Capitan) doesn't like the icon all of a sudden.
I tried emptying the browser cache and dumping Google's cookies - to no avail. It doesn't cause a functional problem, only an esthetic one.
Any ideas?
