For what? Firefox DevTools? I use Firefox, spent several years as a Web dev, and use FireBug pretty much every day, so I think I'm part of the core demographic. That's unrelated to my use of
other browsers though: right tool for the right job, and all that.
The parent just-so-happened to make a tangential comment about "all browsers" bundling dev tools, which I found quite short-sighted. If you're QA testing a commericial Web site then by all means use "all browsers" as a shorthand for "the popular browsers". But if you're commenting on the general development of browsers and their features, such statements need to be qualified. After all, there are some good sibling comments discussing WorldWideWeb, Netscape, etc. Should they be dismissed as "edge cases"?
Maybe instead of listing the software I actually use, I could have taken a guess at how many mobile browsers lack dev tools; I imagine there are many users of such browsers, which is far from an "edge case", although I avoided making such conjectures since I've never browsed the Web on a mobile phone.