When you click the little thumbnail image on the toolbar, it should focus/activate the Firefox window. If you can't see the window anywhere, one of two problems comes to mind:

(1) The window is on a different desktop (if your Windows has more than one desktop)

(2) Firefox has positioned the window off the screen in error

One possible way to get the window on the current display is, after you activate it using the taskbar thumbnail, to press

Alt+Spacebar

This may cause a visible window control menu to appear in the corner of the screen. If it does, click Maximize and if it doesn't, type x to maximize the window. Any luck?

Even if that works, the next new window might be off-screen, so it is worth investigating further.

Thank you but it didn't work

If you right-click the Taskbar button and click Open new window, does it show up on your current screen?