Hi!
And welcome to the forum!
Sadly only Safari, Chrome and Brave are supported by the Front Browser actions and tokens. This has to do with Firefox apparently not supporting any scripting (irritatingly as I myself is used to Firefox as my main browser. Hoping in the future Orion will be natively supported by KM as I am about ready to jump ship!)
This is the method I use to grab Firefox' front window, current tab, URL:
