It's due to the use of the blur backdrop-filter which seems to have been enabled by default on release builds of Firefox relatively recently. Interestingly, it

It's due to the use of the blur backdrop-filter which seems to have been enabled by default on release builds of Firefox relatively recently. Interestingly, it's a feature introduced to CSS way back in 2015. There must have been some major blockers on getting that out the door if it took ~7 years.