This is not for the developer of the page to solve.
While biking back home, a possible cause popped up in my mind: could one of the add-ons I use cause this and yes, that appears to be the case.
I restarted with disabled add-ons and the message was gone.
Then I enabled them one at a time and the culprit is
Selenium IDE.
A bug report on this issue was closed with Won’t fix, with the message:
This error will resolve itself when we move to a native app later this year.
In a MozillaZine topic of 2012, it is explained how it could have been solved.
The first one is just a warning that the addon is using “Components”
directly, which won’t necessarily always be possible when using the
Add-on SDK. (The preferred way to do it is to access the aliases for
Components.classes and Components.interfaces and such that the SDK
provides by requiring the “chrome” module.) It shouldn’t be a problem
right now, but might become one in the future.