I won't add a "default on" mode. That should not be needed - most sites do work fine without spoofing as Chrome, and doing a spoof by default will actually expose you to all kinds of breakage. In addition, the Web Compatibility team at Mozilla (of which I am part of!) wants to actively know about sites that DO require us to spoof as Chrome, which is why the addon itself provides two easy ways to report. A "default on" option would not be the right approach.

Thanks for the feedback about the visibility of the indicator. It's kinda hard to find the right balance here, but I have a couple of ideas to try to make it more visible if this comes up more frequently. I'll keep you in mind!