SYMPTOM
JavaScript with alert method cannot work in the HTML container in Report Service Document when running in Express Mode in Firefox.
STEPS TO REPRODUCE
Follow the steps below to reproduce the issue with Strategy Tutorial project:
1. Create a Report Service Document and insert a HTML Container.
2. Add below code in the HTML Container (HTML Text):
<script>
alert("A");
</script>
3. Run the document in Express Mode in IE and Chrome. Note that a window pops up with the text "A".

4. Run the document in Express Mode in Firefox. Note that no window pops up.
5. Run the document in Interactive Mode in IE, Chrome and Firefox. Note that a window pops up with the text "A".
CAUSE
This is a known issue in Strategy Web 9.4.1
ACTION
This issue has been fixed in Strategy 10 presentation mode. Take advantage of upgrading for a fix.