It's very much the kind of alert you'll see if you run a command with no open document.

Any idea which extension might be triggering it?

So it's a recent problem, eh? I'd try the usual: Quit the application, see if that helps. Quit all other applications, see if that helps. Shut down and restart your computer, see if that helps.

If you really suspect the extension, you can easily uninstall it (and reinstall it later). Once you've ruled out a number of possibilities—or found a direct correlation with the extension—then it may be time to contact the developer about it.

As far as similar problems, I recently opened Fireworks 8 to test the File Management button (the Put command in particular) and found that it triggered an alert message normally produced by Aaron Beall's "Deform to Path" command:

I tried closing the Path panel, and the alert message was still triggered. (I do have that command also available within the Commands menu, but I'm not sure what was going on there; however, since I don't use Fireworks 8 much anymore and never use the Put command, I decided to leave it alone.)

Currently, I use Fireworks CS6. Several months ago, I noticed a "migrating panel" bug, where panels within the default configuration had been mysteriously swapped for one another: The Color Palette panel had been replaced by Lorem Ipsum, while the Path panel had gone missing, replaced by the Color Palette panel—bizarre behavior, to say the least. I tried reselecting a default Workspace (as well as a saved one), but it had no effect on the problem. I still haven't taken the time to diagnose and fix the issue, but I suppose it might have somehow been triggered by the Lorem Ipsum panel, which was a custom installation.