Fix crash on exit (BL-487)

The main crash got fixed in geckofx, but this change fixes some
missing dispose calls that might also have contributed to the
problem.
Right. TemporaryFolder will delete the temporary folder in its Dispose() method. But we didn't call Dispose() on it and had a comment saying that we don't want it to be deleted since that might cause problems. Which means there is no point in using a TemporaryFolder object.