xulrunner can't load plugin in standalone app on mac

Hi,

I create a multiplatform XUL app with xulrunner embedded. 
The app is just a browser and a plugin.
I define the xulrunner profile in a specified location (near the app) and I put the plugin in the profile/plugins folder.

On Windows (XP and >), the plugin was loaded and it run perfectly.
On mac OSX 10.7, the plugin was seen (using about:plugins) but can't be loaded.
If I launch the XUL app using Firefox on mac, the plugin is loaded correctly.
If I use the standalone version, plugin can't load.

Do you have any idea why?

regards,
Phil