I had my L5 docked and Firefox open in full screen. I did something to shrink the browser window to smaller than full screen. I could not find a control you would find on a desktop machine which causes a window to fill the screen ( not F11 type full screen mode, just normal maximize window ). You know the big rectangle and small rectangle at the top right of the screen that switches to a rectangle, or a diamond shape. The minimize/maximize controls.
Best I could do was reside the windows by dragging the corners.
You can open it via a terminal and add some flags, this way you can easy open firefox with a custom size:
$ firefox --window-size 1024x768
But like on every Desktop, if you have a Mouse at your Docking station, try to double click on the Main-Bar on Top of your Firefox. Like the space right of your Tabs. This Way it should maximize or minimize it by Mouse.