[KomoD]

It proxies it through their servers. You are connecting to Nitter, not Twitter. They are serving the content to you (the user), not Twitter.

[xigoi]

Alright. If someone sends child porn to someone by physical mail, is the mail company responsible for it?

[stemlord]

Twitter would be the party mailing the letter to Nitter, and Nitter is mailing it to the end user. Nitter is responsible for mailing it to the end user. It doesn't matter that they originally got it from Twitter or aliens from Uranus. The mail company is Firefox browser.