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There is a bug in the firefox 3 beta 3 "customise toolbar" dialog, which causes a number of problems:
* Updatescan icon cannot be added to the main toolbar
* Various menu text disappears when the "customise toolbar" dialog has been opened and closed

This is a known bug in firefox 3 beta 3 (https://bugzilla.mozilla.org/show_bug.cgi?id=414907). Apparently it has be fixed for firefox 3 beta 4.

Firefox 2 is not affected.