>>> My Info: Win7_64, Nightly 49, 32bit, ID 20160526082509 STR_1: 1. Open attachment 8702617 [details], click the button on the page to switch to DOM fullscreen 2. Press Ctrl+T STR_2: 1. Open attachment 8702617 [details] in a new tab 2. Open http://example.org in the next tab 3. Switch to the tab from Step 1, click the button on the page to switch to DOM fullscreen 4. Press Ctrl+Tab AR: Another tab briefly appears before the screen becomes black (during fullscreen transition) ER: Another tab shouldn't be visible until the end of transition (otherwise why do you even have it?) It appeared after bug 1160014, but it's rather bug in a new feature than regression. Regression range: > https://hg.mozilla.org/integration/mozilla-inbound/pushloghtml?fromchange=cc263dfb7b136f8719eed944932d860cf71e8ac1&tochange=b867bb9c50ee758113bf288a2baca283226382e1

Interesting. I saw this only once on very first attempt with latest Nightly 54.0a1, 20170124030205 on Win 7. I did not see it at all on latest Nightly 54.0a1, 20170124030205 on Win 10.