Have icedove co-installable with xul-ext-foxyproxy-standard

As of Tails 1.1rc1 tails ship xul-ext-foxyproxy-standard 4.1-1 from experimental. This package prevent you to install icedove:

Package: xul-ext-foxyproxy-standard
Source: foxyproxy
Version: 4.1-1
[…]
Breaks: iceape (>> 2.17~a1), iceape (<< 2.0~a), icedove (>> 20.0~a1), icedove (<< 3.0~a1~pre), iceweasel (<< 3.1~b3)

apt-get install icedove xul-ext-foxyproxy-standard
[…]
xul-ext-foxyproxy-standard : Breaks: icedove (> 20.0~a1+) but 24.6.0-1~deb7u1 is to be installed
E: Unable to correct problems, you have held broken packages.

Parent Task: #5663 (closed)

Original created by @foob on 7587 (Redmine)