Firefox windows work generally the same way as other windows, which is that you can only resize them when they are in a restored state, not when they are maximized. Use the center button of the three at the upper right of the window frame to toggle between restored and maximized.
If you are in full screen mode, floating over the upper right corner for a few moments should make the buttons appear.
If you cannot see the lower right corner of the window frame because your window is too big, you can copy the following script and paste it into your location bar and press enter to resize the window to a smaller size.
Hi redlake2, what happens if you try the script I posted above? It should reduce the Firefox window to 25% of the screen so you can move and resize it. If you don't have the handy grab handle on the lower right, check View > Status Bar to restore it. Does that help?