Resizing the window is a function of the computer. At the top-right of the window are three boxes. The first box, an underscore, drops the window to the taskbar. The last box, an X, closes the window. The middle box changes the window between full and small screen.

Have a look through the thousands of Firefox addons

• https://addons.mozilla.org/firefox/search/?q=window+size
  • Window Size Fixer https://addons.mozilla.org/firefox/addon/window-size-fixer/ Provides ability to fix the initial size of the main window.

I have not tried that or any other in that search listing, but it may do what you want.

Please try it and post back to say if it works ?

Hi John99, Thanks a lot for you help. I was exactly what i was looking for. With that add-on you can define the size of the window at the pixel. That's great. Regards, FH

FredMcD said

Resizing the window is a function of the computer. At the top-right of the window are three boxes. The first box, an underscore, drops the window to the taskbar. The last box, an X, closes the window. The middle box changes the window between full and small screen.

Hi, Thanks for your answer but it is not what i need. I don't need to learn to use of my operating system. Regards