README.md: add web-app fix for Firefox users

Author: Zimrahin

README.md

@@ -76,6 +76,10 @@ a list of available DotBots, allows to set which one is active and controllable
 and provide a virtual joystick to control it or change the color of the on-board
 RGB LED.
 
+**Firefox users:**
+If the webapp is not working, press `Ctrl + L`, type `about:config`,
+and set `network.http.http2.websockets` to `false`.
+
 ### Lighthouse positioning
 
 The DotBots firmware comes with a cheap indoor positioning system based on