Download fails for me too. Here's what I tried but the end result still reads as versions 12 and not 13.

# cd /var/cache/apt/archives/partial/
/var/cache/apt/archives/partial # ls
fennec_13.0_armel.fennec.FAILED
/var/cache/apt/archives/partial # dpkg -i fennec_13.0_armel.fennec.FAILED
Package fennec has package signature (origin 'com.nokia.maemo/ovi')
(Reading database ... 51898 files and directories currently installed.)
Preparing to replace fennec 12.0 (using fennec_13.0_armel.fennec.FAILED) ...
dpkg: warning: subprocess old pre-removal script returned error exit status 1
dpkg - trying script from the new package instead ...
dpkg: ... it looks like that went OK.
Unpacking replacement fennec ...
aegis-installing fennec (from 'com.nokia.maemo/ovi')
Setting up fennec (12.0) ...
Processing triggers for desktop-file-utils ...

/var/cache/apt/archives/partial # dpkg -l|grep fenn
ii fennec 12.0 Web browser