Waaaay too much false information (or lack of) by the author and respondents here.If people would read before installing things they would be aware of this. It is right out in the open in plain comprehendbale wording (not bunched together like a legal doc). So anyone that took the time to read the small paragraph would be fully aware of what's going on, or at least know they will be guinnea pigs for testing out the new tech...

#1 If you download the Developers Edition, Beta or Nightly edition, you agree to share crash data, new feature rollouts and participate in sheild test (which you can still opt out of). They tell you flat out these things will happen by participating in these previews (Which what this addon was part of and was tested by a measy 1% of the user base)

#2 By the screenshot provided, this user has "Legacy Extentions" enabled, which means they have to be using a Nightly build, as that is the only way you can still enable this feature. Hence they did not read the details on the possible trials they might be joined in. If they were part of say the Electrolysis trial I doubt there would be concern like this.

#2 In a full public release you are opted out of these by default, but can opt in if you wish.

#3 If you download the official release you are not subjected to any of this unless you decide to manually opt-in. It's not hidden, it's on the privacy page in the main preferences. For the pioneer experiments you have to go manually download a full extention to opt-in.

#4 If you downloaded the beta to check things out, then when 57 rolled out and you decided to stay on the release channel, you would have to manually go and switch the experimental pioneer off manually or refresh your profile as it keeps all your custom profile data. Same if you shared a single profile between all your installs instead on creating multipile profile.

#5 Firefox keeps a list of which experiements are on the way, details about it and you can see which ones you are taking part in in your about:support and also in about:studies

Info about this study taken from their study page: https://www.dropbox.com/s/vu2n2llbfyyyv1x/Screen%20Shot%2020...

Info detailing how to opt-in: https://www.dropbox.com/s/sk48o3fgookin4i/Screen%20Shot%2020...

Proof of my default settings on Nightly which opts me in my default: https://www.dropbox.com/s/blar0u0jdrzw37b/Screen%20Shot%2020...

An finally proof you are opted out by default on the public release of 57: https://www.dropbox.com/s/3zpwunknngy71pw/Screen%20Shot%2020...