My up-to-date Firefox Portable began giving me a Crash Reporter pop-up error window every time I close FF normally.
So I went to an older FF backup folder, a clone copy that's a few months out-of-date b/c I intended it that way, and started it instead as a test; and immediately after closing it normally, it also gave me a Crash Reporter pop-up error window.
That implies something external to FF Portable is going on, but what?
So then I went into my up-to-date version of FF Portable, and renamed crashreporter.exe to crashreporterDISABLED.exe, and restarted it, then closed normally, and this time no Crash Reporter pop-up window appeared.