So, if there isn't an option why does View --> Font -- > Font Window have a checkbox for "Set as default"?
About ten years ago, when there wasn't a "set as default" option in the fonts dialog, users asked for a "set as default" option for different types of windows.
I added it, but because of the way the [font] settings in mirc.ini are not network-specific, "set as default" would apply to all types of certain windows (such as status windows) by default, so it was disabled in the dialog.
That is, I believe, the entire history of the "set as default" option :-]
I don't think anyone has mentioned it since it was added about ten years ago.
If I understand correctly: if the checkbox was "checked", as well as being disabled, that would have made it clearer? That makes sense. I'm going to make that change in the next version.