Evolution doesn’t seem to follow the system rules when opening web links.
First check what is the current setting, and see what other browsers are available:
gvfs-mime --query x-scheme-handler/http gvfs-mime --query x-scheme-handler/https
Change the default to Firefox:
gvfs-mime --set x-scheme-handler/http firefox.desktop gvfs-mime --set x-scheme-handler/https firefox.desktop
