Excel forces "Number format" dropdown selection into Accounting upon pasting anything, also randomly formats all empty cells in worksheets to same

[Henwayracing]

I've manually converted all "number format" selections in every worksheet to be General or Custom or text or whatever. I've rooted out all "Accounting" format. Yet what I save it or paste into Excel from other sources like emails and directly from Firefox, Excel forces it into Accounting when it's not (I always have to paste-as text). How can I prevent this nefarious and prolific Accounting format from infiltrating my workbook/worksheets?