I used to use Link Widgets for a long time, but sadly, it does not work anymore with Firefox 43.0, and for the previous version neither. I'm not sure since when it does not work anymore.
The buttons still appear, when hovered, the link is well displayed, but when clicked, nothing happens. I found the following message in the browser console:
TypeError: 'shiftKey' getter called on an object that does not implement interface XULCommandEvent.
Also, I didn't find any appropriate place to communicate this bug.
I hope this add-on is still maintained, because I find it really useful, and I'm not aware of any alternative.
Edit: The patch suggested here (https://addons.mozilla.org/en-US/firefox/addon/link-widgets/reviews/487218/) works well, but it breaks the add-on signature. It is only a temporary workaround.