From MozillaZine Knowledge Base

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 are using a different Firefox theme, switch to the default theme (Tools -> Add-ons -> Themes -> Default -> Use Theme). [1]
• If you are using a non-standard operating system theme, try switching back to the default one.
• A user reports problem was fixed by turning off the third-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

External links

• Window shakes (Firefox Support)

Related bug reports

• Bug 178884 - Bookmarks-toolbar-part with folders can get into a resize-loop when on the menubar
• Bug 247116 - Web pages bounce/shake when "Bookmarks Toolbar Items" is empty and placed to right of menu bar
• Bug 448844 - Pages shake vertically when "Bookmarks Toolbar Items" shows nothing