[Bug-gnuzilla] installation old version failure

[message]

Hello,

I'm using an old mandrake distribution and trying to install icecat 
version357 
(http://ftp.gnu.org/gnu/gnuzilla/3.5.7/icecat-3.5.7-i386.tar.bz2)

It is not possible to install plugins due to the following error:

GNU IceCat could not install the file at

https://addons.mozilla.org/firefox/downloads/latest/1964/addon-1964-latest.xpi?src=dp-btn-primary

because: Signing could not be verified.
-260

Selection of the appropriate certificates to activate the option to 
allow software makers, fails to solve. Any other suggesions please?