See http://kb.mozillazine.org/About_protocol_links

OK... so why does a search for about: on this site still not find any matches when the subject line of my original message consists of precisely those letters?

Too bad the about:about shown on that mozillazine.org page doesn't work in Firefox.  :-|

That page specifically says:
about:about — Lists all these about: URLs (Mozilla Suite/SeaMonkey only).

That about: protocol link doesn't apply to Firefox.

I never said it doesn't say that.

I said, Too bad the about:about shown on that mozillazine.org page doesn't work in Firefox

i.e. because that was one of the first variants I tried before I started looking for a list of them, yet found nothing about them in its Help nor on this 'support' site.

MozillaZine has been around since Mozilla was founded about 11 years ago and has had a lot more time to build a database of support articles. Many of those support articles cover SeaMonkey, and Thunderbird (both of which used to be official Mozilla products, but were handed off to volunteer groups so that Mozilla could concentrate on Firefox and the core Gecko code), along with Firefox. It can be confusing at times for newer users, with all that information intermixed. Up until this forum was started all Firefox users were referred to MozillaZine.

This forum is about 3 years old and doesn't have as mush user support as MozillaZine has developed, so building the support database will take some time.

BTW, about:about was added to Firefox 4, along with many more "about:" protocols as actual hyperlinks in about:about.

There is also about:memory to see Firefox's current memory usage (doesn't work on Linux with Firefox 3.6).