> As an aside I always kind of wondered why all browsers started including developer tools by default as they are not of use to 99.9% of users.

AFAIK the firefox devtools team has plans/ideas that the devtools could just used like a normal Web App (loaded from a server). Currently, they are migrating XUL based components over to normal html what would make this possible.

As an example, just like debugger.html which can even be used from any browser in a not-builtin way: https://github.com/devtools-html/debugger.html