okay i figured it out, you have to create a .desktop file for miru in .local/share/applications and it must contain this line:

MimeType=x-scheme-handler/miru;

you can use Gear Lever which adds Appimages to the applications menu and creates the .dekstop file automatically for you so you don't have to do everything manually even the line above, it also has an update mechanism too

it's preferable to Temporarily use Brave or a chromium browser as the default for it work properly if Firefox is your default