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?

I assume you mean "from my web browser", since you could also be accessing it via say Apple Mail.

That little thing is called a favicon, and most web sites have them. Here's MFIF's:
https://www.finetunedmac.com/favicon.ico

Those are NOT cached AFAIK by any browser, because they can change. I don't know why it wouldn't be rendering, but clearing your browser's (image?) cache would seem to be the way to fix that.

Just out of morbid curiosity, when you see that icon missing, try removing the rest of the address and put in "favicon.ico" on the URL and see if it loads?
(like https://mail.google.com/favicon.ico )

I did, and it didn't.


The itty-bitty favicon appears in Firefox's tab when Gmail is open. And the same version appears when following your directions.
The larger version (in upper left of all windows when viewing email) does not.

Further to previous discussion:

The (itty-bitty) favicon [envelope with superimposed red M] which appears in the Firefox tab weighs in at 1.62KB / 32px x 32px and has location designated by https://ssl.gstatic.com/ui/v1/icons/mail/images/favicon5.ico .

What does not render — as noted earlier it shows up as grayed-out outline with broken icon on the left and text Gmail by Google to the right — is the larger icon [same content + associated text Gmail by Google] which appears on each window in upper left corner is 3.07KB / 143px x 59px and has location https://www.gmail.com/mail/help/images/logonew.gif .

Both icons show up under Page Information; obviously both load.

So the difference would seem to be in the extension — here the (static) GIF not rendering where it's supposed to. I don't usually have any problems with GIFs.

Any further thoughts on resolving the problem?

Since it is a cosmetic issue you might report it to Mozilla and hopefully it will be fixed in the next release or the next or...

Grelber, I can confirm that it's NOT a problem with Firefox Quantum 59.0.2 OR FirefoxDeveloperEdition 61.0b1 (64-bit) on High Sierra 10.13.4 as I just tried it - so not sure what is causing the missing icon for you. It seems odd that it would be OS-related. Are you running the most recent version of FF?

So - the mystery deepens a bit - when I check that icon, I find it here: https://ssl.gstatic.com/ui/v1/icons/mail/rfr/logo_gmail_lockup_default_2x.png - totally different location than you're seeing, and a PNG, not a GIF. How odd... ???

My bad. I should have noted that I'm running the latest Firefox Quantum 59.0.2. I'm also using Mac OS X El Capitan 10.11.6 with all updates.

'Tis a poser. I don't understand how the affected icon would be differently stored with a different extension if we're both using Gmail.

I've cleared Firefox's web content cache, shut down and reopened Firefox, as well as closed and reopened Gmail. So far that has all done bupkis.

Addendum:
All the other icons render in the appropriate places (eg, print, new window, starred).
Correction, re the large Gmail icon: Envelope rectangle with superimposed red M with the word Gmail to the right.

I just updated Firefox Quantum to version 60.0, and the Gmail icon issue (non-rendering) persists.

https://www.gmail.com/mail/help/images/logonew.gif
Size: 3.07 KB, 143px × 59px

I have found that reporting bugs to Bugzilla is fraught with frustration.

After intermittently futzing around with various settings in Firefox Quantum (now 60.0.2), it turns out that FF's tracking protection is the culprit (contra Paddy's observation). That .gif image was determined (by FF) to be involved with tracking and was disabled automatically. When I disabled the tracking protection on that site, the image reliably shows up.

Now the question becomes: Is it worth enabling FF's tracking protection for Gmail just to be able to see the image on my Gmail pages when the possibility exists that the image is somehow involved with tracking activity by Google given that Google is everywhere and tracks everything?