Usually Firefox knows not to cover the Windows Taskbar except in full screen mode. To switch between normal and full screen views, you can use the F11 key.

If Firefox is not in full screen view, there are two different display modes: maximized and what Windows called restored or resizeable.

• If Firefox covers too much of the screen when it's maximized, this can indicate some corrupted settings. Is that the issue?

• If Firefox covers too much of the screen when it's resizable, you will need to resize it down a bit. If the lower right corner is visible, click it with the mouse and drag leftward/upward to reduce the size of the window and then release. If that corner is not visible, you can use a keyboard resizing trick, which is as follows:
  • Press Alt+spacebar to drop down the control menu
  • Press the letter s to select size
  • Then press the right arrow key once to indicate that you want to change the size on the right side, then press the left arrow key repeatedly until the window is the size you want, then finally press Enter to lock in the change

Any improvement?

As near as I can tell (if I followed your instructions correctly) I ended up with no change. I also thought that I might try a restart to see if that helped and It would not restart. It will hibernate but not restart.

Any ideas ?

One quick way to switch between a maximized window and a resizeable window is to double-click the title bar (area above the tabs). Does that do anything?

If the tabs go all the way to the top of the screen and fill all the space, then there's no place to double-click. Then you'll need to use either the "middle button" between the minimize and close buttons, or Alt+Spacebar and r for Restore.