For dev environments, the devtools: true option is very valuable. Since it is Chromium-only, the current behavior is to throw an error when the option is passed for Firefox or WebKit. This means the user needs to tweak the launch args whenever the browser type is changed.

Assuming this option cannot be applied to all browsers (needless to say that would be awesome), I think Playwright should throw a warning and continue with the browser launch. That way, the same code can be used across the 3 browsers, making things a bit simpler. Possible warning text:

devtools is not supported as a launch argument in Firefox. You can launch
the devtools window manually.