Thank you for letting me know of such issue. Unfortunately I can't seem to reproduce the issue on any of my machines and different versions of Firefox. But after clicking the blue trashcan icon it should display a pop-up with a confirm remove dialog. As a workaround while I can't solve the issue, please use the tab-context menu "Add/edit entry to Blacklist" option to remove or add a page to the Blacklist or Whitelist.