If your Firefox window shakes, bounces, or jumps up and down erratically, you ​can ​try ​the ​foll​owin​g acti​ons ​to addr​ess ​the ​problem.

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• Check extensions and themes
• Check toolbars

Check extensions and themes

• If you recently switched to a new theme, switch back to the default one - see Customizing Firefox with add-ons.
• If one of your extensions or themes recently updated, try disabling it to see if it is the cause. For instructions, see Troubleshooting extensions and themes.

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 for more information.
• To reset all toolbars and controls to the default settings, follow the instructions to Reset Firefox toolbars and controls.

Based on information from Window shakes (mozillaZine KB)