Have you checked the troubleshooting done in this topic? That user had broken icons symlinking, done during installation.
Although this is clearly a Gnome upstream (potential) issue and should be posted there, you might want to check possible caching issue (~/.cache/), or local config, which is strangely ~/.config/thunderbird/, as the developer says in an AUR post.
It doesn’t seem to be an application/installation issue, rather a DE or system configuration (not EnOS I suppose).
Maybe another Gnome user could confirm or not. 