reduce default screen height for facebook etc?

[boof]

messages [firefox] ends r off screen, too low to read. i can't see how to raise the bottom of the window. other apps allow it, why not default ff? i'm sure someone will know. i'm using 1920x1080 image, magnified to 57in panasonic plasma tv screen circa ~2012? do not know screen resolution, prob > 3800x2100, 16:9-not in my documents.