Why can Thunderbird be there but not Mozilla?

[Tuxito3000]

Why does Thunderbird appear in the game but Mozilla can't be added due to copyright? They're from the same company, why?

[Alayan]

SuperTuxKart has a blue bird for the start countdown and rescues that's inspired by Thunderbird, but the name "Thunderbird" or logos similar to those trademarked by Mozilla don't appear anywhere.

[Sauss-Ente]

This really invites for some satire designs! May I present to you an idea, Wozilla, a big bloated dinosaur, which has a arm-tattoo showing the letter "G" in red-blue-yellow-green striped colours. Its kart has many sensors (at each wheel, at the bonnet and the rear end) and also an antenna on top. The sensors and the antenna start blinking on the winning and losing animation, meaning that the kart is now collecting data and sending it somewhere.

Also, Wozilla Jr. A smaller version of the big guy, but sleek and elegant. Same kart model, but more simple and angular. Also a turquoise "N" appears on the kart as a sponsor. But most importantly no sensors and antenna. Very important detail.

[Tuxito3000]

SuperTuxKart has a blue bird for the start countdown and rescues that's inspired by Thunderbird, but the name "Thunderbird" or logos similar to those trademarked by Mozilla don't appear anywhere.

I got confused, they were very similar to each other