What happened?!

I was merrily going about my business and put my iMac to sleep (as usual). Two minutes later apparently it decided to shut down completely on its own (while in Sleep mode).
I only discovered this some 2 hours later when I had to start up from scratch.
The only diagnostic report information is in the form of the following 2 lines:

12-06-27 8:24:28.877 [0x0-0x1a01a].org.mozilla.firefox: NOTE: child process received `Goodbye', closing down
12-06-27 8:26:44.309 powerd: PMScheduleWakeEventChooseBest: m=0.00 s=0.00 a=0.00

I have no idea what either of those lines means.

More important, is this an indication of something having gone catastrophically wrong with my iMac? And is it likely that I'm going to experience more unpleasantness due to whatever caused the shutdown?

FYI:
Running Mac OS X Lion 10.7.4 on iMac 2.5 GHz Intel Core i5.
I have since verified the disk and repaired permissions, with nothing apparently "out of order".

EDIT (1): Apparently whatever happened caused my browser (Firefox 13.0.1) cache to clear.

EDIT (2): Since I use dial-up Internet access, the computer is not connected to same at any time I do not actively engage it.

And I ran an updated anti-virus program (Sophos) on all files (including archived and compressed): no threats detected.

So I'm still trying to figure out what went wonky. Not a peep from assembled talent.

What technique did you use to put the Mac to sleep?

4 of out 5 times via the power button; 1 out 5 via the Sleep command in the Apple menu. In this case I imagine it was the former.

If the symptom occurs intermittently, it will be hard to diagnose. In the meantime, it wouldn't hurt to reset PRAM.

So far it's only been that once.
I am extraordinarily reluctant to reset PRAM unless it's absolutely necessary.
And it would hurt: I hate resetting all my preferences.

So ... no one knows what those 2 lines from the diagnostic report mean?

Maybe you won't have to reset too many preferences. What's Stored in PRAM.

I've seen this happen twice, once on my own machine, both with the same cause. Turned out to be a battery that had become possessed. It would apparently just stop sending power to the computer, at random times. I'd be using it or it would be awake, and then would CLICK off like I'd pulled the power cord without a battery. It started becoming very frequent when I would leave it awake overnight, I don't normally sleep it overnight so the backup runs. And I kept getting up to see my computer was OFF.

Took me awhile to narrow down the cause. I had a second battery and it never happened with the other one. Replaced. Problem solved. There were no other reasons to suspect the battery, it showed healthy with a low 100's cycle count and good capacity, and passed all tests.

As far as I'm aware, the only batteries associated with my set-up are in the Magic Mouse, and that wouldn't have anything to do with the problem.

Sort of. I have no idea what caused that shutdown, as your Console log entries don't explain much if anything. Neither line is all that rare, but an explanation of an associated shutdown is. Apparently Firefox quit due to an external issue like a shutdown or (aborted?) restart process, somehow associated with the sleep process, or rather the awakening from sleep (referred to in the second log entry containing 'powerd').

As I mentioned above, there are a number of error reports from users running Lion about restarting when putting a Mac to sleep (unknown causes), or about not sleeping at all (running background processes) featuring one or both of your two Console log lines (FWIW, I haven't uncovered another shutdown like yours). Like the latter, the former is probably associated with some software based process, instead of with hardware faults, but that rather vague 'conclusion' is about all I can glean from those (otherwise mostly inconclusive) reports, plus the lack of hardware problem reports.

Are you talking about a PRAM-battery equivalent, or the power battery of MacBooks? Grelber runs an iMac, so the latter wouldn't apply to his situation.

Merci.
I guess that's where we're going to have to leave things for the time being.

A side note which might or might not be relevant: I have seen this happen (a computer shutting down while it is asleep) if a kernel panic occurs while the computer's asleep or while it's in the process of going to sleep.