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https://bugzilla.mozilla.org/show_bug.cgi?id=1249151 — bug to reintroduce cookie prompts;
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https://bugzilla.mozilla.org/show_bug.cgi?id=1235199 — for SeaMonkey, but contains a patch that 'reverts the parts from [bug] 606655 for [Internet] suite.'
The cookie prompt has been a very important privacy feature in Firefox and derivatives, because it allows people to choose whether to accept cookies from a domain or not. People trained on this can also tell apart domains that are generally ok, and those that are not.
Upgrading to 45 and greater deletes all previously accrued cookie permissions, thus resulting in data loss and broken sites.
The only recourse has been to use Firefox 38.x ESR, because it has so far retained the cookie prompt functionality.
-Mart.