[FIREFOX] We sometimes flag cookies as "secure" even though they are from HTTP origins
While investigating this bug report I realised that HTTPS Everywhere will actually flag cookies as secure from within HTTP-only pages/origins. Needless to say, this can interact very badly with *.example.com target host rules!
This is something we explicitly avoid in the handleSecureCookies() path that deals with cookies set in HTTP headers.
But we have another path which I think was added to fix #3766 (moved) which does not perform the same check.
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