As others have mentioned several fonts can be used within a single text run due to font fallback. Since the font matching code is platform specific, the logging is different on each platform.
Under Windows, font matching logs can be dumped even from production builds. Open up a command shell and set the following environment variable:
Change directories to the one containing Firefox 3 and run the executable directly:
You'll see the details of all font matching operations spewed to a console window on a character-by-character basis.
On the Mac you'll need to set up your own build and uncomment the // #define DUMP_TEXT_RUNS in gfx/thebes/src/gfxAtsuiFonts.cpp. Rebuild, then open up a Terminal window and use:
Each text run will be dumped with a record of the fonts that were matched.
Beyond simple logging that works on all platforms we should probably have a way of inspecting this information so that Firebug can display this information directly.