Can't login when "delete cookies" preference is enabled. #1429

Trying to login results in "Something went wrong" page, without any explanation of what.
According to the account menu, I'm already logged in.
It doesn't matter if I access Send from the account menu or type in the url.

Unchecking the delete cookies preference, or adding send.firefox.com to the whitelist results in the expected behaviour.

Related?:

• https://bugzilla.mozilla.org/show_bug.cgi?id=1429714 a
• Issue#1226