Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:62.0) Gecko/20100101 Firefox/62.0 Build ID: 20180619220118 [Affected versions]: Nightly 62.0a1 [Affected platforms]: Windows 10 x64, Ubuntu 16.04 x64 [Steps to reproduce]: 1.Launch Firefox with a new profile 2.Go to https://mozilla.github.io/webrtc-landing/gum_test.html 3.Click "Window" or "Screen" button 4.Click to open the Share dropdown [Expected result]: The drop-down with the available screens/windows should be displayed [Actual result]: The Select screen is duplicated in a new option line and is highlighted [Note]: The issue is not reproducible on Mac OS X 10.13

The issue is reproducible also on Firefox Release 60.0.2 and Firefox Beta 61.0b14.

That sounds a little like a difference in standard behavior for Windows vs. Mac. If it's a bug on our end, I wouldn't say it's a bad issue.