Fabian,
Yes, the designer war that works for you is an older version that should work for all browsers. The only downside is that that version is still using an older version of the BPMN2 specification. Version 026 is more recent but only works on Chrome (some strange javascript loading issue). We're currently updating to the latest version of the designer, so we will be updating that soon with a newer version using the latest BPMN2 specification and supporting multiple browsers again.
Kris