There is a sudden exploit that has been found in Mozilla Firefox. It has been patched already but you may need to manually update. In your menu at the top click on Help and choose About at the bottom of the list. If it doesn’t say version 74.0.1 then update. Having 74.0 is not enough, make sure that it’s 74.0.1 and you’re good to go.
[UPDATE]: Any version at or ABOVE 74.0.1 is good. Last I checked we’re now at version 75.0.
Stay safe and wash your hands! ?