Firefox is my usual browser, but when I try to reproduce this problem in Chrome or Edge it does not occur. Thus it's presumptively a Firefox issue. I'm using FF 72.0.2 64-bit in Windows 10. Some of the cells in my spreadsheet have a green background. This is NOT because Fill Color is set, and it's not tolerable, because I use Fill Color to represent the status of a cell. Once a cell "goes green," nothing I have tried will fix it. Today I decided that I had to fix a particular row containing green cells even if I had to re-enter it. I created a new row above the problem row and copied the original row's cells to the new row one by one, pasting each one into the text box at the top of the sheet, rather than the cell itself. That should have inserted the text only. But as I entered the text into each new cell in turn, the cell turned green. I need to find a solution to this problem, or I'll have to change browsers.