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1.5.0.3 is the present stable. i sense some muscle in this community that they have such good releases so tightly dated.
@sprite; that default.xxxxx folder is openable & contains a sub-folder called extensions, you could save that before you do an uninstall ( as you could bookmarks & search plugins from said folder ).
But the preferred method is to install the new release atop the old one, that way you keep all relevant userdata. Remember to update the extensions from the extension tab in options after updating firefox itself.