Oh, I thought you'd got Fireworks working for GIF files. In that case I suspect you just need to go to Preferences / Double-Click / Files and either turn off Use internal picture viewer for... or change it to unregistered picture file types only.
When you first installed Opus the wizard will have asked if you want Opus to handle double-clicks on picture files and that's what initially determines this setting.
Edit: Just remembered it is also covered by this FAQ.