Since I believe FMGO already uses safari for the webviewer, it won't let you launch it externally since it overrides the HTTP and HTTPS protocols inside of the web viewer object itself.
Alternatively, you could possibly create a "link handler" script, that when executed via the fmp:// protocol, would run an Open URL script step.
So your links would change to look something like:
and your linkHandler script would look something like
Open URL ( get(scriptParameter)) no dialog
Unfortunately, this would only work if you have control over the code being displayed in the web viewer, to be able to code those links accordingly.
Thank you Mike, this solve my problem.