If you encounter some applications freezing or taking a long time to start while using i3, I think in particular about Skype, Discord, CMST (Connman QT GUI), Hexchat, … then may be here is a solution for you. Those app do not appreciate when the desktop does not have any notification daemon running. So you might just install and start one when i3 starts.
So I’ve just installed lxqt-notificationd. Then you can start it from i3’s config file:
exec --no-startup-id lxqt-notificationd
Now all my applications work like a charm, and I’ve no reason whatsoever to temporarily switch to XFCE4.
Today I was surprised to see a GTK-3 application opening an HTTP URL with Opera. I don’t use Opera and I just installed by curiosity long ago and forgot about it. I configured the Debian alternatives however GTK-3 seems to use xdg-mime as confirmed with an strace of the concerned application and references to /usr/share/applications/defaults.list. Note that you may have to create a symlink for defaults.list to /usr/share/applications/mimeapps.list.
You can use the xdg-mime command to configure the default application for each protocol: