The restore button will re-use the last window size, so it appears your Firefox has remembered a nearly-maximized size. If the lower right corner of the window isn't hidden (behind the Windows Taskbar or offscreen), you can click and drag on that corner to shrink the window. If you're lucky, Firefox will remember the new size. If not, you might have to do it every time. (It's a little unpredictable which window applications use to store the size information.)

Could I mention something else I discovered accidentally? If your computer has a keyboard with the "Windows Logo" key, that has some interesting shortcuts. If you hold down that key and press the left arrow key, the current window will be resized to the left half of the screen. Similarly with the right arrow key, the right half of the screen. This is a quick way to line up two windows side by size at the same size. (Up arrow maximizes, and down arrow restores.)

I did the reducing the screen with the lower right corner as suggested and it fixed it immediately!! Thank you!!!!