I followed the instructions Getting Set Up exactly and it doesn't work.

And the instructions are confusing. Suddenly, firefox is mentioned, which has already been started and of which there was no talk before.

Step 4. Start the debugger

Now that Firefox is open, let’s start the development server. In a new terminal tab, run these commands:

windows 10 64 bit 1903

D:\TiddlyWiki\My_TiddlyWikis\git\debugger>yarn start
yarn run v1.19.1
$ node bin/dev-server
Listening for WS on localhost:8116, all traffic is proxied to localhost:6080
Protocol messages can be logged by enabling `logging.firefoxProxy` `in /configs/local.json`
Hot Reloading - https://github.com/firefox-devtools/debugger.html/blob/master/docs/local-development.md#hot-reloading
View debugger examples here:

https://github.com/firefox-devtools/debugger-examples
Development Server Listening at http://localhost:8000
 
webpack: Compiled successfully.
 

And yarn startalso messes with the internet connection.