If you are working with multiple different versions of Firefox, this add-on might help you, as it always displays the version you are currently using. Because there is too little space on a toolbar button to display long version numbers, there are two add-ons available that complement each other. This add-on displays the minor version. To see the major version, install the separate addon "Version in Toolbar". If necessary, customize the toolbar to reorder the two buttons so that they are next to each other, the button showing VER on the left, the button showing SION on the right...
