To reproduce:
1. Open attached project "VI_one_scene_exp.zip"
2. Build for WebGL
3. Navigate to project folder Assets > StreamingAssets and copy ME_scene_1_30.mp4 video to WebGL build location so it's in the same folder as index.html file
4. Run the index.html file on Firefox
5. Observe as the video looks like it is stopped even though the audio is still running

Expected result: video runs smoothly without stuttering or freezing
Actual result: video is freezing even though audio is still playing

Reproduced on:
2018.3.0a6 2018.3.12f1 2019.1.0b10

MacBook Pro (15-inch, Early 2011)
2 GHz Intel Core i7
8 GB
AMD Radeon HD 6490M 256 MB
Intel HD Graphics 3000 512 MB

Notes:
-Tested with Mozilla Firefox.
-Can't build for 2019.2 versions. Getting this in the console:
"Plugin 'nunit.framework.dll' is used from several locations:
Packages/com.unity.ext.nunit/net35/unity-custom/nunit.framework.dll would be copied to <PluginPath>/nunit.framework.dll
/Volumes/External/Applications/2019.2/Unity 2019.2.0a1/Unity.app/Contents/UnityExtensions/Unity/TestRunner/net35/unity-custom/nunit.framework.dll would be copied to <PluginPath>/nunit.framework.dll
Please fix plugin settings and try again."
-Doesn't reproduce on late 2013 iMacs and MacBooks or later.
-When opening the project on 2017.4 2018.3.0a5 the project breaks. Missing prefabs in the hierarchy.
-Didn't reproduce on Win10 machine