I got emailed a pointer to a development build
right after I posted my note above, and that worked fine
I've now installed the current release as of yesterday or the day before, and that works fine.
The changelog text first line shows it as
Adblock Plus 0.5.8, but the "About" window shows it as 0.5.8+
Mozilla/5.0 (Macintosh; U; PPC Mac OS X Mach-O; en-US; rv:1.8b4) Gecko/20050725 Firefox/1.0+
---> note the last few nightlies sometimes require a restart of the Mac before Firefox will show Adblock or anything else added to the Tools menu -- not just a restart of Firefox, but a reboot of the computer, to get the Extensions menu to be correct after dropping a new nightly build in.