Where did you download Firefox from?

From mozilla itself. It was the official download.

HEY! I just posted abt a problem similar to this too except my white bar bordered the left, right, and bottom sides of the screen but I've been looking at so many different questions abt this BUT i found this link https://support.mozilla.org/en-US/questions/1302185#answer-1345901 there's a message answer by Philip the moderator that fixed my problem mine was doing the same thing as urs. If it doesnt work at first unselect all the settings and then close ur browser and open it again. If it still doesn't work click the "Change Settings for All Users" button make sure everything is unselected there too and then hit apply again and I also ran the compatibility troubleshooter too and clicked enable suggested settings too I think it said and did that so that could've done it as well? Not sure but one of these things should work for you fingers crossed!!