Android's Clear Data button completely deletes your Firefox settings files. I am not aware of any way to undo that unless there is an "unerase" tool for Android files as there are on desktop operating systems. Some ways you might have made a copy of those bookmarks: * Did you ever set up Firefox Sync and sync the bookmarks to a Firefox Account? * Did you ever use the [https://addons.mozilla.org/android/addon/copy-profile/ Copy Profile add-on] in Firefox for Android?