Hi Claude,
thanks for making the HP file available to test on.
As you suspected, Firefox was handling the onload javascript slightly differently to other browsers, and as a result FF was running the HP StartUp function twice, giving you double the number of questions.
I have fixed this issue now in HotPot (Moodle 1.9 and Moodle 2.0) and QuizPort (Moodle 1.9).
BTW, because of the recent change to use GIT for the Moodle repository instead of CVS, I can no longer update the HotPot module for Moodle 1.9. Until I work out how to install and use GIT, you can get the most recent version of of the HotPot module for Moodle 1.9 from here:
thanks for bringing this issue to my attention
Gordon