Comment 7 for bug 73244

The helper application could also have no need to keep the file open all the time, for example by reading and "digesting" it all at once. But then, in case it wants to access it again for any reason you can think about, and Firefox was closed in the meanwhile, here the problems come.

Confirmed also with Firefox 3.0rc2:
  Mozilla/5.0 (X11; U; Linux i686 (x86_64); en-US; rv:1.9) Gecko/2008052912 Firefox/3.0