xdg-open uses diffrerent app in-sandbox and outside

[Roman Evstifeev]

Hi list!
I installed Slack app from falthub, and it opens http urls in the
SeaMonkey browser, instead of my systemwide-configured Firefox. And
this command indeeed launches SeaMonkey: "flatpak run -d
--command=xdg-open com.slack.Slack http://google.com"

Is it possible to configure flatpak so that sanboxed app honor file
and url associations of the host?