They're the same browser though.

They actually aren't the same browser, since MS rips out some stuff and adds their own. Including features I like tree style tabs.

It's a similar and derivative browser, but they aren't the same.

There are more similarities than differences.

You can say the same about tons of things, but the differences are what matters. You and I share more similarities than differences, just like the browsers, but we aren't the same person.