KM browser tokens are defined for Safari, Chrome, and variants thereof which use the same osascript interface.
token:WebBrowserTokens [Keyboard Maestro Wiki]
They can't be defined for browsers, like Firefox, which don't provide an osascript interface, and are thus beyond the reach of JavaScript for Automation and AppleScript.
Arc appears to use the Chromium engine, so it may appear to Keyboard Maestro to be a version of Google Chrome. See discussion here, for example: