Bug#634053:

[Robert Keevil]

I also have this exact same issue.

trident:/tmp# sha1sum /var/cache/apt/archives/xulrunner-dev_5.0-3_amd64.deb
34798069440e351365d5f77bc731cd063533bd3e
/var/cache/apt/archives/xulrunner-dev_5.0-3_amd64.deb
trident:/tmp# ls -l /var/cache/apt/archives/xulrunner-dev_5.0-3_amd64.deb
-rw-r--r-- 1 root root 4525032 Jul 15 06:32
/var/cache/apt/archives/xulrunner-dev_5.0-3_amd64.deb

matches the info on http://packages.debian.org/sid/amd64/xulrunner-dev/download

I use the US mirror, but have tried several others.