Well, the web font responsible for displaying the icons is not applied.
I’d recommend checking about:config for gfx.downloadable_fonts.enabled = true and other font-blocking settings.
Setting gfx.downloadable_fonts.enabled to false shows the same behavior as in your screenshot.
Also, you can use the ‘Font’ tab in the network monitor (Menu > Web Developer > Network) to see if the ‘MaterialIcons’ font is loaded.