Like the other recent reviewers, I've found this fab add-on has been broken by changes in firefox 50.
Such a shame. I've had this literally for years.
Mozilla had been faffing around with ALT+ENTER behaviour (which I'd allocated to adding the .co.uk suffix) for a while, which caused issues with this add-on's behaviour (it opened a new tab, though the suffix still worked).
Now, Mozilla have clearly done something more radical.
I hope the author can find the time to update it.
I've given it 5 stars, despite it not working, simply to thank the dev for producing such a functionally useful add-on.