If your Firefox window shakes, bounces, or jumps up and down erratically, you can try the following actions to address the problem.

Check extensions and themes

If you recently switched to a new theme, switch back to the default one - see #How to switch themes.
If one of your extensions or themes recently updated, try disabling it to see if it is the cause. For instructions, see Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems.

Check toolbars

If your Bookmarks Toolbar is showing but it does not contain any bookmarks, either add a bookmark to it or hide the Bookmarks Toolbar. See Bookmarks Toolbar - Display your favorite websites at the top of the Firefox window for more information.
To reset all toolbars and controls to the default settings, follow the instructions to Reset Firefox toolbars and controls.