Hi Firefox does not have a "one-click undo" button for a data import. 1. If you just started using Firefox and don't have much data, the fastest way is to '''Refresh ''' Firefox. '''It restores Firefox to its default state while keeping your essential data like bookmarks and passwords'' More info [[Refresh Firefox - reset add-ons and settings]] 2. It’s easier to just create a new profile. * Type about:profiles in your address bar and hit Enter. * Click Create a New Profile. * Name it "Your name" and finish the wizard. * Under the new profile, click Launch profile in new browser. * If it looks good, you can go back to about:profiles and Remove the old, profile. More info [[Profile Manager - Create, remove or switch Firefox profiles]] 3. If you have existing Firefox data you want to keep, you’ll need to delete the imported items manually e.g History: Ctrl + Shift + H You can sort by "Date Added." Since you just imported them, you can select the bulk of these entries and delete them.