Window shakes

If your Firefox window shakes / jitters / bounces / jumps, try the following solutions:

Update Firefox to the latest version.
If your Bookmarks Toolbar is showing but it does not contain any bookmarks, add a bookmark to it or hide it (View -> Toolbars -> uncheck Bookmarks Toolbar)
If you put your Bookmarks Toolbar Items on a different toolbar (such as the File/Edit menubar) and have hidden the Bookmarks Toolbar itself, remove the Bookmarks Toolbar Items (View -> Toolbars -> Customize -> drag the Bookmarks Toolbar Items back to the Customize Toolbar window -> click Done). Now, replace the Bookmarks Toolbar Items to where you want it.
If you recently switched to a different Firefox theme, switch back to the default one (Tools -> Themes -> Firefox (default) -> Use Theme)
If you are using a non-standard OS theme, try switching back to the default one
A user reports problem was fixed by turning off the 3rd party application "Windows Blinds" (MozillaZine forum topic).
if you have the extension eQuake Alert, disable the option to shake the screen when a new earthquake is reported.
Try the standard diagnostic

Related bug report