Younglings may not remember, but Mozilla browser used to include a kitchen sink[1], as a joke on the number of functions that were added to it as an internet suite beyond being just a browser[2].
There is still about:robots though, which has been maintained and updated in the new style over the years.
And of course, about:mozilla is still present.[0]
[0]: The Beast continued its studies with renewed Focus, building great Reference works and contemplating new Realities. The Beast brought forth its followers and acolytes to create a renewed smaller form of itself and, through Mischievous means, sent it out across the world.
[1] https://www-archive.mozilla.org/docs/web-developer/samples/k...
[2] https://bugzilla.mozilla.org/show_bug.cgi?id=122411