Sorry to reply to my own post, but I may have tracked down the problem, and perhaps others can benefit from what I found.
I noticed that breakpoints were working fine, on the same code, in my Windows 7 versions of Firefox + Firebug. I have far more extensions in Mac Firefox (my primary devel environment) than in my Windows copy. On a hunch, I turned off FirePHP (an extension for Firebug, which I only had installed in the Mac version) from within its own popdown menu. Didn't help. Then I disabled FirePHP completely via my Firefox Extensions list, and relaunched Firefox. The problem seems to be solved: the breakpoints in Mac Firebug now behave normally.
So... it seems there may be a conflict between FirePHP and Firebug breakpoints. Unless, of course, it was a coincidence (since the problem was intermittent to begin with). Anybody else notice this conflict?