Here's a longer explanation. https://www.brycevandyk.com/dissecting-firefoxs-no-remote-op... By default, Firefox looks for an already running instance and attaches to it, opening a new window in the existing session. --no-remote disables that connecting behavior.

By default if the browser is running, the command you run will use RPC to pass your instructions to this browser. This maybe isn't what a seasoned Unix person would expect software to do but it matches expectations from many GUI users.

The --no-remote flag tells Firefox that no, you really want another Firefox.