We might be able to use the browser_style thing for this. https://developer.mozilla.org/en-US/Add-ons/WebExtensions/manifest.json/browser_action

(I don't know if it only affects a panel or the button too)

I think @ianb added the browser_style thing to addon/webextension/manifest.json in #2377:

"browser_style": false

Although the I believe false is the default value. So sayeth the docs:

Although this key defaults to false, it's recommended that you include it and set it to true.

browser_style won't do anything, I'm afraid, it just applies to panels. There isn't a panel, so I added it just to make a warning shut up.

Note though that this ticket is for the favicon, not the toolbar, so this is entirely a website issue. Maybe we want a background on our favicon?

If this bug is about the colors in the favicon, that doesn't seem like a beta blocker. Removing that label; feel free to re-add it if I'm missing something.

Since the new Firefox Compact themes will enter in Firefox 54 release, I have also checked how the Screenshots tab favicon behaves on them. It seems that we have the same issue described above, using "Compact Dark" theme.

Here is a screenshot with Screenshots tab favicon on Compact Dark theme: