To clarify, this is an issue with Facebook. The bar in your screenshot is not the Firefox address bar, it's a part of Facebook's website.

That said, it should stay at the top of the page.

Many site issues can be caused by corrupt cookies or cache. In order to try to fix these problems, the first step is to clear both cookies and the cache. Note: This will temporarily log you out of all sites you're logged in to. To clear cache and cookies do the following:

1. (Firefox 57 and above) Click the Library button and choose History.
   (Click the menu button and choose History in older Firefox versions.)
2. In the History menu, choose "Clear Recent History...".
3. Under "Time range to clear", select "Everything".
4. Now, click the arrow next to Details to toggle the Details list active.
5. From the details list, check Cache and Cookies and uncheck everything else.
6. Now click the "Clear Now" button.

Further information can be found in the Delete browsing, search and download history on Firefox article.

If that doesn't help, try running Firefox Safe Mode to ensure that Ghostery or your ad blocker are not breaking the Facebook website.

Hope this helps.

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"