Need help with Warning: Use of Mutation Events is deprecated

[Pim]

I need help with my website. The Error Console says

Warning: Use of Mutation Events is deprecated. Use MutationObserver instead.

but I have a couple of problems with that.

First of all, the Error Console doesn't say where on the page these mutation events are used. So where do I look?

Now it would help if I knew what Mutation Events actually were, but I haven't been able to find out exactly. When I type "what are Mutation Events" into Google, the answer universally is, Mutation Events are deprecated. Use MutationObserver instead.
Right. Now one page, MDN says that there is a list of all mutation events, DOMAttrModified, DOMAttributeNameChanged etc; OK, but none of those words appear anywhere in the files on my site!

So what can I do?

[Frenzie]

Mutation events do what the MDN page says. They allow you to attach events to DOM changes. Except they're horribly slow and ineffecient and you should use Mutation Observers instead.

Did you search locally in your own development files or through all active script files using e.g. Firebug? And don't forget about its log events feature. Because it might be from some ads or trackers some such.

[jscher2000]

Possibly an extension (easy to bypass in Firefox's Safe Mode)?

Do you use any scripting libraries on your site, or only your own code?

[Pim]

I found it! I found the culprits!
Oh dear, this is embarrassing.
In my defence, let me explain that this is an old website, you know. I just inherited it recently and I've been working on it to make it error-free, mobile-friendly and so on. But so far I haven't actually removed any, ehm, features. The owners of the website like it just the way it is, so they don't see the need to remove any functionality.

Oh well, I'll just have to redo the damn things in Javascript then.