Check the resolution. Modify > Image size.
Thank you. I lengthened it from 1200x695 to 1250x695. I don't know why it needed to be changed, I thought 1200 fit my screen. Ah well.
The problem was that my document size wasn't adjusting to the screen size, although it seems that before it was. Isn't there some kind of option to make your layout format itself to any screen size no matter the length and width?
IMHO 1200 or 1250 pixel layouts are too wide for web pages. You need to take into account that most people still are using 1024 x 768 pixel screen resolutions, and they don't necessarily view web pages with their browser windows maximised. You also need to reduce the dimensions by 20 pixels on each side to adjust for browser chrome. A good size to work with is 960 pixels wide.
You can create fluid layouts, but that needs to be done in Dreamweaver or whichever HTML editor you are using. You can zoom in and out of the layout in Fireworks, but the actual pixel domensions remain static.