I am already actively developing the next version of this add-on, which uses different technologies and therefore isn't affected by this. That's why I didn't notice it.

This is caused by the inconsistent pathnames Firefox uses for content scripts. You will see that it works for all pages that are ope at the time you enable the add-on, but not for pages loaded afterwards. The exact format of those paths must have changed fin FF53.
I opened https://github.com/NiklasGollenstede/wikipedia-peek/issues/9 to address this in a hot-fix.

Edit 2017-05-01: Version 2.7.3 has been submitted for the code review.