http://www.edmondscommerce.co.uk/firefox/firefox-duplicate-tab/

If you are careful about which extensions you use, remove the extensions compatibility check…

• Type about:config into Firefox’s address bar
• Right-click anywhere. Choose New>Boolean. Make the name of your new config value
extensions.checkCompatibility and set it to false.
• Make another new boolean pair called extensions.checkUpdateSecurity and set the value to false.
• Restart Firefox.

Whenever you go to Add-ons, you’ll see a notice reminding you.

Another approach is to edit the extension’s installation file. Extract the XPI and edit the file called “install.rdf”:

3.1a1pre