Quick reply...
Download deb to N900 from, for example,
ftp://ftp.mozilla.org/pub/mobile/rel.../binary-armel/
or
ftp://ftp.mozilla.org/pub/mobile/rep.../binary-armel/


Then open the folder in File Manager and open the deb (with App Manager). If it refuses to install the deb, read the log: what kind of problem is here?

You could also try (as root) the command
dpkg -i /path/file.deb




About the contradiction between "dpkg -l fennec" and "apt-get purge fennec": dpkg has one list of packages, apt-get has another. You can perfectly well see with dpkg that all the applications, which you have uninstalled someday, have left behind their config files, settings, version, etc. Don't worry about it; it seems to be the norm.

Best wishes.