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You think this should have been a built in function of FF.

Howto: Fixing NewTabURL Rated 5 out of 5 stars
by GGITech on September 23, 2012

Remove NewTabURL
If you have done any of the about:config / browser.newtaburl changes, then undo them.
browser.newtab.url - should be blank
browser.newtabpage.enabled - should be true
Add NewTabURL.
Now the Add-On works as expected, including the "URL from Clipboard"