ash3T I don't think this can be done since it's a native browser confirm() box.
Ultimately we will probably replace all those native alert() / confirm() calls with custom Flarum modals that can be extended. We might have an issue for this somewhere but I can't find it. We have this open issue for a different modal we want to make customizable flarum/core2054
The particular case of the composer exit might be trickier than other alerts because it can also be triggered by closing the tab/window, in which case native actions might be the only available option to us, since browsers don't allow blocking the tab close without showing a native message.
