Will there be a "always on by default" option?

[denschub]

No. While some people request a "default on" option, that's not going to happen.

1. Spoofing as Chrome by default breaks a lot of websites. A lot of sites run Chrome-specific code if they detect Chrome, and this will fail in Firefox. Frequently, web developers don't expect those kinds of errors, so you frequently just get a blank page or a completely broken feature. It's not a good experience.
2. I want encourage users to report meaningful reports to Mozilla if a site needs a Chrome-spoof to work. This won't happen if the addon is on by default.
3. You will get locked out by a lot of sites, and you will see a lot of CAPTCHAs. These anti-bot providers are really good at "detecting weird traffic", and they can detect Firefox as Firefox even if you use this addon to look like Chrome. If sites get "misleading" signals, they'll frequently consider you bot traffic, and thus might block you.

For all those reasons (and technically even more), a "on by default" mode will not happen.