Firefox 'accessing graphics' error...

[andyoxon]

I keep getting a Firefox error saying 'firefox has been blocked from accessing graphics hardware', often following the screen going dark for a second or two - during regular browsing.   This win10 PC has onboard intel HD 620 graphics; 7thG Core i5. 

Anyone had this, any ideas?  Bin firefox?

[Kim]

Is some website trying to mine bitcoins in the background?

Thinkig about it, it might be a security or driver problem, rather than a Firefox one.

[andyoxon]

Thanks.  I use Windows Defender, so maybe.  Intel graphics driver' released Jan 2018, updated May 2019', so should be OK.  Glary says 'no spyware'.