Screen shot of the error and what toolbar are we talking about?

mischkah said

Now my toolbar is missing.

Would you please try this :

3-bar menu => Options => Customize => Toolbars

Does that do the trick  ?

Make sure you do not run Firefox in Full Screen mode (press F11 or Fn + F11 to toggle; Mac: Command+Shift+F).

• https://support.mozilla.org/en-US/kb/how-to-use-full-screen

When you are in Full Screen mode, hover the mouse to the top of the screen to make the Navigation Toolbar and Tab bar appear. You can use one of these to leave Full Screen mode:

• press the F11 key
• click the Maximize button at the right end of the Tab bar
• click the Full Screen button in the "3-bar" Firefox menu button drop-down list
• right-click empty space on a toolbar and click "Exit Full Screen Mode"