Thanks for the review and letting me know! I'm not able to reproduce this issue but I'll add a failsafe for it anyway. Are you perhaps on a slower machine or using old or beta version of Firefox?

EDIT: Check now, I pushed an update that could resolve it. I couldn't reproduce the issue so no promises.
EDIT2: In response to your update: So that was the case! I'll look into restoring icons with unread count.