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). 
- 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
- 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
Retrieved from "kb.mozillazine.org/Window_shakes"
Category: Issues (Firefox)