It means that we are sponsoring those tags on Stack Overflow, have a landing page there and that people at Mozilla actively monitor and take part in discussions marked with that tag.
Do you know why Mozilla's Stack Overflow tags are using the dino head logo instead of the fox?
Yes, I do. :-) That logo is the new logo for MDN, our network for developers at Mozilla. Since Stack Overflow is a resource for developers we see a direct connection to that. Also, we have different foxes for different products, so that would probably be confusing - rather have one consistent look.
I could barely recognize what that tag icon means. A small odd white shape on another odd blue shape. You can spot the Google Chrome and Android icons from a mile. Come on.