User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:37.0) Gecko/20100101 Firefox/37.0 Build ID: 20150312193711 Steps to reproduce: 1) have FF with one or two tabs open 2) press Ctrl-T to open a new tab Actual results: The new tab opens, but in about 1-5% of cases, the screen does only get updated in a very small region around the new tab marker. The URL bar and the main content window do not get updated. See attached screen shot. This happened first with the FF 36 beta 1, it still happens with beta 5. Though this happens to me multiple times a day, I haven't yet found a way to reproduce the issue reliably. Expected results: Proper screen update with correct display of "about:newtab" (which is configured to be blank).

- Not only when opening a tab, also when closing a tab - I observe this on multiple machines (Windows 7, Windows Vista)