If the installation does not start, in Firefox a yellow notification bar should appear at the top of the page. You should follow the instructions on that bar, and try to install the extension again.
When no yellow notification bar appears, you should make sure the "Allow web sites to install software" checkbox is checked (Tools -> Options... -> Web Features).
After installation, you must restart the browser to load the functionality of this extension in the browser.
Duplicate Tab 1.0.2 is compatible with the following products and versions:
2.0 - 3.1a1pre
1.1 - 2.0a1
The following localizations are included in Duplicate Tab:
Firefox tends to crash when Duplicate Tab and EZ Sidebar are installed together (reason is unknown yet)
Firefox / Seamonkey 2.0a1:
This extension can be uninstalled in the Extensions Manager (Tools -> Extensions). Select the Duplicate Tab entry and click uninstall.
Please, install the ExtensionManager, to easily uninstall Duplicate Tab.