Twitter has shut down the platform’s legacy theme on June 1, 2020, forcing desktop users to use the social network’s new mobile-inspired layout. GoodTwitter2 (developed by Bl4Cc4t) will re-arrange Twitter’s new interface to make it look good again. Users will get the legacy navbar back, you will have your Twitter profile back on the left side of the interface, and you’ll no longer have to use mobile layout on your desktop. Firefox users will have to completely disable Content Security Policy (CSP) by going to about:config and changing the security.enable flag to false.
Source: https://www.bleepingcomputer.com/news/technology/how-to-get-the-old-twitter-layout-back-if-you-hate-the-new-one/