This is probably due to a change made to Firefox with the intention of helping support of High Definition displays.

You can try an add on that may help see

• see Webpages are large and blurry after updating Firefox - how to fix
• use https://addons.mozilla.org/en-US/firefox/addon/nosquint/

If that does not work well and you have a mis match with icon etc size and webpage size then follow this advice:

Firefox on Windows is now following display scaling options which may render text larger on high resolution displays. You can adjust your screen resolution. To modify it do the following:

• Type about:config into the Firefox address bar and tap on the Enter key.
• If the warning This might void your warranty appears, click I'll be careful, I promise.
• Search for layout.css.devPixelsPerPx

• Double-click on layout.css.devPixelsPerPx to modify its value. The default is -1.0 in Firefox 22 and above. Change it to 1.0 to make it work like in previous Firefox versions.

If necessary, further adjust the value in 0.1 or 0.05 steps. Use values between 1.0 and about 0.5 to reduce elements in size. Use values greater than 1.0 to increase size. For example, a value of 1.25 will increase font size to 125% to accommodate the default DPI setting in Windows 8. Double check the value that you enter. Setting a too small value will make everything disappear and a too large value will blow things up.

If web pages still need to be adjusted then you can look at the Default FullZoom Level or NoSquint extension.

To adjust the font size for the user interface, you can use the Theme Font & Size Changer extension.

I tried adjusting the value of layout.css.devPixelsPerPx but that did not help. The only thing that temporarily fixed my display issues was to close Firefox- at that point the screen blacked out and reappeared in full view. As soon as I open Firefox the screen blacks out and reappears showing only a portion of my desktop and all the icons not visible are no longer clickable because the mouse cursor can't reach them.

Right click the Firefox shortcut on your desktop, select properties, and uncheck something like "640 x 480 mode". Hit apply, and open firefox, does it work now.

You can open the Properties of the Firefox desktop shortcut via the right-click context menu and check the "Compatibility" tab.
Make sure that all items are deselected in the "Compatibility" tab of the Properties window.

• http://kb.mozillazine.org/Screen_resolution_changes_when_launching_application

Everything under compatability was already unchecked.