It sounds like you might have put Firefox in full screen mode. You should see an option the Firefox menu for "Full Screen" - you can select this to exit full screen. If not, you can also hit the F11 key on your keyboard to achieve the same thing.

You may also have maximized the Firefox window. If that is the case then undo the maximize and possibly resize the window by hand.

This works temporarily only. If I open a page where I need to scroll down on an internal screen, ie. the emails giving the responses above, as soon as the arrow on the scroll bar gets to the bottom the task bar disappears. If I go to the "triple bar" menu and click the full screen and then click it again to leave full screen then the task bar comes back. But it disappears again if that scroll bar is used. It is Ok if only the scroll bar on the far right at the edge of the page is used, so is only internal. Have tried it three times to get it to disappear on the internal scroll and then cycling the full screen on then off.