Out of interest:
How come FF ERS has returned?
There are already threads on this topic, here, here, and here.. In those you will find links to Purism articles that explain the issue.
In summary, Purism is no longer supporting PureBrowser in favor of a move to Epiphany. It has added Firefox ESR to its repository in PureBrowser’s place. There is an article to explain how to migrate one’s settings.
(Oops. Sorry @kieran, I read your reply as a post, not a link.)