I've had Thunderbird instaled on this Windows 10 machine for about 18 months, and it has been running fine. A couple of days ago, a strange grey bar appeared along the bottom of the Thunderbird window, with red test "toolbarbutton" - see screen shot. I am not sure why this suddenly appeared, there were no unusual events at the time. It just showed up, one day. Any suggestions how I can remove this extraneous artefact. I tried a lot of add/remove toolbar, change layout, etc, through the View menu, nothing made any difference. I also trawled through prefs.js but didn't see anything obvious. Very grateful for any assistance.