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I Used FF Until They Screwed Old Extensions With The Introduction Of Version 4,
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As I remember, this is also my last FF version.
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Related to your "problem" with duplicate Tabs... I would suggest that you consider solution "seen from a different angle"... instead of closing, why don't you try this extension
(it's for Chrome) which prevent opening of duplicate tabs - "
Clutter Free":
https://chrome.google.com/webstore/detail/clutter-free-prevent-dupl/iipjdmnoigaobkamfhnojmglcdbnfaafAs it works, the site or page that you’ve opened on your second tab will be automatically closed, leaving the current tab as an empty tab. Apart from that, the extension will immediately redirect you to the first tab.