Hello,
Yeah, this was pointed out just recently to me.
I will add some reset button that will clear everything because yes, reinstalling won't reset data, those are stored in Firefox Sync engine.
But there is a workaround:
1) open popup
2) right click anywhere and select "This frame" / "Open frame in new tab"
3) go to that tab and press "Ctrl + Shift K" (this will open console window)
4) write there:
browser.storage.sync.clear()
5) and press Enter
That will reset all data including old password.

I will try to release an upgrade before the holiday that will add a proper reset button.

Best regards,
Juraj Mäsiar