I had an irritating issue of google spreadsheets showing screen corruption when any cell was clicked. This issue started after the sheets update in which they introduced the "Explore" button on the bottom right.One workaround is to click on the small downward arrow to the right of the column headers A, B, C, D etc. When the column menu comes up and we dismiss it by clicking again on the column header, the sheet seems to get cleaned up. But only temporarily. Waiting for Firefox or Sheets to get this fixed.
Edit: 6 Jan 2016 - this seems to be fixed now.