Showing the ID somewhere in the UI has been part of my wishlist for a while. I have filed an issue with all details needed to implement the feature; follow the feature request at https://github.com/Rob--W/crxviewer/issues/82

PS. This is a personal project from me, provided free of charge. Please consider that demanding a feature in an entitled way is very rude.

EDIT: If you were looking for extension IDs of an already installed add-on, look at about:support#extensions-tbody