
Firefox 28.0, NoScript 2.6.8.24. I am certain the issue stems from NoScript. The bug disappears in browser's safe mode or with only NoScript extension fully disabled, but it remains if I Allow Scripts Globally. I have google-analytics.com blocked, but nothing changes if I white-list it (all the other scripts for Google Sheets are already whitelisted). I have all notifications enabled, and none appear when I load a spreadsheet or try to resize borders. Nothing NoScript related in browser console either.
I can see network exchanges in browser console when I drag borders, and if I reload the page the changes are retained and displayed properly, but not until then.
Don't know if it's a recent issue, I have barely used new Google Sheets since their introduction half a year ago, but I don't recall having such issues when I tried new Sheets several months ago.