For almost every page I open in Firefox, I see this error in the Console of the developer tool bar:
(!) The Components object is deprecated. It will soon be removed.
The source is the html page. It happens with pages I create, but also on many common websites.
I found this documentation on Components object on MDN web docs, but that does not clarify a lot. Note that even that page shows this message(!)
It looks like a warning, but according to the Console filter, it is an error.
My main questions are:
I am not aware of any problems as a result of this. For now, that is.
I have seen this for over a year, maybe longer. I mostly ignore this, but every now and then it starts nagging me again. I don't want my code to break suddenly and would like to get rid of this message obscuring other messages.
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.