Ok, I've found why the posbox does not work in firefox. It is due to the "Mixed Content" Security Policy, which prevent loading http ressources from a page served in https. In other words if your point of sale is served over https, firefox will block the http connection to the posbox.

There are two practical solutions :
1) Serve the point of sale over http. (this will be done on our saas platform)
2) Deactivate mixed content security in firefox. This can be done in about:config. The security.mixed_content.block_active_content setting shout be set to false