Now that extensions require user permission to do some things, users may reasonably expect that if their permission is not requested, an extension cannot do a lot of things. But that isn't the case with experiments. We need to add a generic permission and/or warning for all extensions that contain experiments. Perhaps we have some useful text from previous versions.

I have the code to do this mostly figured out, but I'm not a fan of the wording, which I took from the warning about unsigned extensions in Firefox, with the first sentence changed.

What I'm trying to tell the user is that although we have an add-ons permissions system, some code in the add-on lives outside it and could do anything without asking for permission. Potentially the "It requires your permission to:" section might not even be there. The "Learn more" link could potentially provide a much more verbose explanation if needed, once it works (bug 1623575).

Edit: I should point out for those not familiar with this prompt, everything except the "Caution: …" part already exists.

Take two: a combination approach. Only the experiment "permission" in the list of permissions and a warning paragraph afterwards which only shows for experiments. I like this one.

What it can do is not limited to things within Thunderbird though, which is what my "Have full, unrestricted access to ..." was trying to convey.
Maybe "Have full, unrestricted access to your system"

That's why I added the longer text below it, which does say an extension can do things outside Thunderbird.

We don't want this message to be too scary, given that most extensions will be using an experiment for the foreseeable future.

Andrei rightly pointed out that we shouldn't show the part about malicious add-ons and trusting the source when the source is ATN, because all add-ons available there are reviewed by humans. With that in mind how about this:

Add XYZ extension?
It requires your permission to:

• Have full, unrestricted access to Thunderbird, and your computer.

(This line only if install is not from ATN.) Malicious add-ons can steal your private information or compromise your computer. Only install this add-on if you trust the source.

I want to keep the mention of Thunderbird in the permission string because it makes the message more reassuring without being incorrect. Also I don't like the word system in this context.

Ok let's go with that.

String changed. Warning about untrusted sources removed for ATN installs.

and that may want a localization note too

Pushed by geoff@darktrojan.net:
https://hg.mozilla.org/comm-central/rev/2373d7011959
Request user's permission to do anything with a WebExtension experiment. r=mkmelin