I've been experimenting a bit and i noticed this:
If in your xml the paths are like this:
<img src="src\globe.swf"/> Internet Explorer is ok with it, but Firefox 3 isn't
If they are like this:
<img src="src/globe.swf"/> then both browsers are fine.
Note the difference between / and \
If you are producing your xml with pdf2megazine, and you type say pdf2megazine.bat T.pdf files\page 60 600 900 into the command prompt, then your megazine.xml will be written using \ instead of /, and Firefox won't find your asset.
If you type pdf2megazine.bat T.pdf files/page 60 600 900 then all is good.
This may not be your problem, but I hope that helps.