Firefox supports setting icons on menus [1]. Chrome refuses to create a menu with unknown keys, so this is guarded by IS_FIREFOX.
This uses the appropriate Breeze icons, except "computer-tablet" which I edited to have larger padding to somewhat fit better with the rest of the icons.

[1] https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/menus#Icons

Probably some ESR testing? Though it said something about Firefox 55, so I suspect that has been in there since.

(Will add the "Share..." icon once D28707 is in)