Hi Richard, I'm happy to hear that people are interested in that feature. The reason I originally wanted to block Ctrl+D was because, back then, Firefox would bookmark the page immediately when Ctrl+D was pressed, and it was very annoying, so I was happy to block it.
If you don't want new Bookmarks to be just dumped, but rather placed where they belong, I advise using Ctrl+B, then drag&drop the tab directly where the bookmark belongs. Still, I'm definitely willing to offer the user the choice of what shortcut to use. I just didn't find a nice implementation that I could reuse.
Yet, this add-on, as-is, is pretty simple, it's just a user script, so it won't detect keys when the page is not selected. Therefore, as a workaround, you could press Ctrl+L or F6 to unselect the page, then hit Ctrl+D.
About my reachability, I guess I'll just publish the add-on on Github and direct people to its issue system.