From MozillaZine Knowledge Base
localstore.rdf stores customized data on the interface, such as toolbar customizations, window positions and sizes, and tree sort orders. In Firefox 1.5 and above, the safe mode option "Reset toolbars and controls" clears this file.
It's not advisable to edit this file manually because of its complexity.
This file can be moved to a different profile without any extra effort.
Deleting localstore.rdf will reset your customized toolbars, window positions and sizes, tree sort orders, and similar settings. A new file will be created on start up or when the application first needs to write to the file.
Before deleting this file, saving a screen capture (print screen) could prove useful in implementing customizations to restore former appearance after this file is recreated.
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