Which in practice would only be executed by the same crowd of people who would have opted-in to the security features on the first pass.

It would be executed by anyone who experiences a problem using a site with Firefox and asks for help about it in a forum, or chat AI, or finds a YT video about it, etc.

Non-technical people do follow recommendations like installing an extension to help with a problem. Especially if they see the same recommendation from multiple sources, and testimonials as to its efficacy.

I don't think Firefox should automatically spoof a Chrome user agent.

There's a long tradition of browsers doing exactly that.

The Chrome user agent starts with Mozilla/5.0, ends with Safari, claims to be like Gecko, based on KHTML, and somewhere in there is the string Chrome.

That was done exactly because people were trying to gate which browsers could or couldn't see their page by name, instead of by missing functionality.

If a site doesn't work, I disagree. It should.

The web is becoming a monopoly anyway... a contradiction to what web once meant.

In practice, it'd probably look like switching to chrome for the majority of users. A lot of tech saavy users would install a plugin but that's a lot of friction especially for random sites or even for an important site. Using chrome or edge would be easier for most users.