Hi

I am sorry, but I am not sure what you mean. Do you keep holding down the power and - volume/zoom button at the same time?

No it takes screenshots whenever I use it.

I also have the same problem with my tablet Samsung Galaxy Tab A7 ( SMT-505),but the problem is that it's not always working. Sometimes the unit restarts,sometimes shuts down completley a tab,but mosly it changes the volume rather thsn a screen shot. So as I asumes it seems that Firefox has bad compatibility ( or that it's a hardware issue), but now matter the facts its completly useless to clam ( do a waranty claim), because that it always come back as "Dealer refuseal". Since no one will look ( do an objective scan), they just do their quick computer based ( factory reset), and nothing more.