Preferred Applications should also modify gvfs-open/gio.
Web Browser of Settings->Preferred Applications impacts xdg-open/exo but not gvfs-open/gio.
The latter (gvfs-open/gio) is used by Evolution, HexChat, and a long list of other applications.
Its active settings:
gio mime x-scheme-handler/http
gio mime x-scheme-handler/https
Changing its settings:
gio mime x-scheme-handler/http <BROWSER>
gio mime x-scheme-handler/https <BROWSER>
Where <BROWSER> is, for example:
*** Bug 10977 has been marked as a duplicate of this bug. ***
Sean Davis referenced this bugreport in commit 8bc59ad1c48df132e917337bd08211be58af5bb1
Configure gio mime defaults with exo-helper (bug #14633)
The above commit adds support for this. Can you please test and confirm it works as expected?