Hi Rose, sorry it took so long but your issue is a known issue in FF, their knowledge base has a fix. I am copying & pasting that here.
Known Issues
This list covers some of the known problems with Firefox 2. Please read this before reporting any new bugs.
All Systems
* Window bounces and shakes. There have been reports in Bugzilla and MozillaZine forums of windows either bouncing or shaking when no items are in the toolbar and in other cases. As a workaround, there are some instructions at MozillaZine that can help resolve this problem.
And here is the link showing other known issues
Known IssuesIf your Firefox window shakes / jitters / bounces / jumps, try the following steps:
* 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
There is also a new version 2.0.0.4
Hope that helps Rose
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