This started happening a few weeks ago. Using Firefox to load Twitter’s webpage would display this text message:

Can’t load twitter : https://twitter.com/ has experienced a network protocol violation that cannot be repaired.

One (rather annoying) way to go around it is to Shift+Reload the page, but you need to do it every single time you load the main stream or just a single tweet. Another, and a more permanent way is to type about:serviceworkers into the address bar, and then search for “twitter” in that page. You should see something like this:

Click the “Unregister” button and you should be good to go.