Thanks,
Firefox ESR 52.0 through Firefox 56.0.2 inclusive, yes? From what I see in Bugzilla, I assume so.
(I’m aware of one case where managed installations of ESR 52.x are probably not the latest e.g. ESR 52.9. I imagine that use cases requiring 53–55 are vanishingly rare.)
In Technical Details on the Recent Firefox Add-on Outage - Mozilla Hacks - the Web developer blog (2019-05-09), Eric Rescorla makes the following observation:
…
- …
- Users of very old builds of Firefox which the Studies system can’t reach.
We can’t really do anything about the last group …
For end users reading that technical post, it may help if Eric or a colleague can clarify (there) which versions of Firefox fell below the Studies threshold.
In the meantime I posted a comment (awaiting moderation) drawing attention to your 52/56 comment above.
Reading https://wiki.mozilla.org/Release_Management/Calendar#Past_branch_dates alongside the December 2016 part of the history at the foot of Firefox/Shield - MozillaWiki, I assume that shield landed (2016-12-21) in Firefox 51.