For most problems SortPlaces will open an Alert window with an email link to send details of the problem to me for investigation.
If the message is "PlacesUtils.history is undefined", and this appears each time you try SortPlaces, then you may have a corrupted Places database. First create a new profile ("firefox -P" from the command line), install SortPlaces and try sorting. If this works then you can fix your old profile as follows:
For other errors you have probably got a bad bookmark. Try using my CheckPlaces extension to look for bad bookmarks and remove them.
If the 'Sorting' window remains on screen for more than a few seconds without closing, then it has failed and there may be an error message in the Firefox Error Console (This is in the Tools Menu).
To see all the messages that can appear in the console you may also have to change some Firefox settings first: Open a new tab with a URL of 'about:config'. Set the following three options to 'true': javascript.options.showInConsole, javascript.options.strict, and extensions.logging.enabled.
Clear the Console and try SortPlaces again. If there are any SortPlaces related error messages on the console then email them to me for investigation.