My Firefox screen page shrinks!!!

[sweater]

I couldn't figure it out why is it that the Firefox page shrinks....:gack: :blink: ...I mean, the lower page goes up and made the screen to get smaller. I already tried the lower right-side arrow to adjust it moving down but it coudn't be move farther to lower down and I couldn't move it more lower to expand the page. :ouch: :P

What's the possible cause of this? ::) ??? :-\