On 2013-05-30 14:29, Elias Persson wrote:
> Seems like a Firefox bug. Should probably be reported to moz.
> Content-Encoding says Firefox should decompress once, with the
> result being something gzipped (according to Content-Type).
>

After some more digging, it's apparently an ancient bug.
https://bugzilla.mozilla.org/show_bug.cgi?id=132702

TL;DR seems to be "don't content-encode anything which
firefox won't be displaying itself".