User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:44.0) Gecko/20100101 Firefox/44.0 Build ID: 20160213030118 Steps to reproduce: This affects iframe youtube and is showing black bars below on status. This affects sites like imgur and other sides that display video using iframe. Example http://imgur.com/gallery/njquI/comment/585726168 (click on youtube video Link) Actual results: if you click on the Video it will show you the video but it will show you a black border below on youtube video. example link http://i.imgur.com/snjuTgH.png On this file SiteSecurityServiceState.txt you will find a line www.youtube.com:HSTS 0 16844 1456007730335,1,0 if you remove that line you will not have these problems but if you play again a youtube video the first time it will play correctly but not on second time. the only way to fix this is to delete www.youtube.com:HSTS 0 16844 1456007730335,1,0 from SiteSecurityServiceState.txt on your Firefox profile. Expected results: It should play the youtube video properly with youtube status bar.

WFM in Fx45.0b4 on Win10.

Yes, it's a known issue with HTTPS and HSTS.

It could be a problem with HTTPS or try with a different browser.