HI, I have the same thing, It maybe the price you pay for using Firefox on a Google Site. I do not think they will fix the issue if there is one at all on their end as per other question you noted.

As you mention the Video is not finished loading, so it may not be a problem at all.

You do have errors in your Video Card Drivers : adapterDescription: NVIDIA GeForce GTX 970 adapterDescription2: Intel(R) HD Graphics 4600 driverDate: 1-23-2018 driverDate2: 10-16-2017 I would suggest updating them both as 1 is 6mo's out and other last year. This may improve the issue or not but because of the errors should be done. failures: [u'CP+[GFX1-]: [D2D1.1] 4CreateBitmap failure Size(240,29510) Code: 0x80070057 x2 of the above.

It also maybe that Firefox does block website tracking cookies for 2secs to increase page load speed. You also have uBlock Origin (as do I) so maybe the combination is causing that issue.

As for turning that off, I do not know. Test with uBlock off.

Good ideas, but neither fixed it :(

AdenFlorian said

Same as this archived issue: https://support.mozilla.org/en-US/questions/1193363 Steps: - open a youtube video on youtube - double click on the video to go fullscreen before the page has finished loading Expected: - it should stay in fullscreen Actual: - it exits fullscreen within 2 seconds

Went to my youtube account loaded BoTW full screen 2:30+ and no exit. I had to hit ESC to go back to Browser mode. So I think something is going on with your FF install.