It is more complicated. The dictionaries are on mozdev. mozdev uses
several, free mirrors around the world for downloads. For instance, if
you click on the download link for the English moji dictionary
http://downloads.mozdev.org/moji/mojien-0.6.20080306.xpi
you don't actually get the file from the server downloads.mozdev.org.
Instead you are redirected to some mirror server, for instance
http://ftp.rz.tu-bs.de/pub/mirror/mozdev.org/moji/mojien-0.6.20080306.xpi
You got your download from a server which rejected it because the
download rate was exceeded (whatever it may mean exactly) It is usually
enough to wait a few minutes until your browser cleared the redirection
page on downloads.mozdev.org from its cache. It will then choose a
different mirror and it will download without errors.
If a mirror does not work you can contact mozdev. There is a mailing
list. See http://mirrors.mozdev.org/ If the mirror frequently runs into
these problems they may have to remove it from the list.
Gerald