I think it's just using an empty PW, but try this anyway.See if clearing cache gets rid of it:- press Ctrl-Shift-Delete (Mac: Cmd-Shift-Delete)- set 'Time range...' to 'Everything'- untick all items except 'Cache'- clear, then restart TBIf that didn'...
You can file compatibility issues- via the 'Report broken site' option (https://support.mozilla.org/en-US/kb/report-breakage-due-blocking) (Send technical and interaction data to Mozilla must be enabled)- on webcompat.com (https://webcompat.com/issue...
Does that also happen in Troubleshoot mode (https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode/)?If not, return to normal mode, then disable all > re-enable one at a time and test
