What is your computer system and Firefox?

Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac=Options) key, and then starting Firefox. Is the problem still there?

When you say the top of the page disappears, do you mean that part of the web page content below the toolbar blanks out, or the whole toolbar area rolls up and off the screen?

If the toolbar area is rolling up and off the screen, Firefox may be switching to full screen view. Please try these:

• Press the Esc key (this cancels full screen view called by scripts in web pages)

• Press Ctrl+Shift+f Command+Shift+f (this cancels browser full screen view)

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

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

Try to rename (or delete) the xulstore.json file in the Firefox profile folder. You can use this button to go to the current Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)
• http://kb.mozillazine.org/Profile_folder_-_Firefox

Start Firefox in Safe Mode to check if one of the extensions (Firefox menu button/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem.

• Switch to the DEFAULT theme: Firefox menu button/Tools > Add-ons > Appearance
• Do NOT click the Reset button on the Safe Mode start window

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes