If your Firefox is not in full-screen mode (i.e. the icon second from the right at the top right of the Firefox window is a while square), but you cant get to the bottom of the window as it is too tall, then you can easily make the window less tall:

Hover your mouse carefully right at the top of the Firefox window until your mouse cursor changes to a vertical line with arrowheads top and bottom, click the left mouse button and drag the top of the window down. Once you have down that you can grab the top of the window and drag it up. If the bottom is then too high above the taskbar, just hover your mouse over the bottom to get the vertical line with arrowheads top and bottom again and drag it until it is just above the taskbar.

When you close and re-open Firefox it should come back as you have set it.

Of course, if you are using a pretty low screen resolution on your monitor, you might find it difficult to view a full web page without using the magnification facilities. You can try holding down the left Ctrl key and using your mouse wheel to zoom in and out with the magnify facility to get the right page content visible.

Hope that helps?

https://support.mozilla.org/en-US/kb/changes-toolbars-and-window-sizes-are-not-saved You may have corrupt xulstore.json or Localstore.rdf file.

Close Firefox. Open your file browser to this hidden location:

• Mac: *~/Library/Application Support/Firefox/Profiles
• Linux: *~/.mozilla/firefox/Profiles
• Windows: %AppData%\Mozilla\Firefox\Profiles
• *C:\Users\<user>\AppData\Roaming\Mozilla\Firefox\Profiles
• Windows XP: C:\Documents and Settings\<user>\Application Data
  \Mozilla\Firefox\Profiles\

Search for the above files. Then rename or delete them. Restart Firefox.