correcting the install.rdf inset

     <Description>
       < em:id>{ec8030f7-c20a-464f-9b0e-13a3a9e97384}</em:id>
       < em:minVersion>1.5</em:minVersion>
       < em:maxVersion>3.*</em:maxVersion>
     </Description>

EDIT: ... btw I do not see where FX comes in, when users want to sign their documents. It is the function of document generators, like Acrobat ... and Acrobat does it anyhow.

Doesn't sound like it is for PDF or XML files.

http://www.si-ca.si/podpisna_komponenta/namestitevProXSignMozilla1_2_9.php
Google translation:

 Installing proXSign XPI for Firefox on Windows add-on code is signed with a certificate issued by a CA-SIGOV, so you need to store certificates have entered the root certificate SIGOV-CA, which should have a tick in the 'This certificate can represent manufacturers programs "(Summary" This certificate can Identify software makers. "). Root-CA certificate SIGOV easier to install by clicking on http://www.sigov-ca.si/sigov-ca.crt. Once you install it, it appears in the list of certificates, which you can access from the Tools, Options, Advanced icon, Encryption tab, the button "View Certificates" tab CSPs (Summary Tools, Options, Advanced tab, Encryption tab, the button "View Certificates" , Authorities tab). In the list of certificate SIGOV-ca found under the "state-Institutions". To the introduction of hooks come to the list of certificates, click on the "SIGOV-ca - state-Institutions", and then click Edit (Summary Edit) below the list. If you enter a checkmark when you install the certificate, you do not have to open the list and look for the certificate. Add-ons for Firefox proXSign install from the website or download it to your computer (shift | right click and Save As ...) and then open the browser FireFox. After installing the add-proXSign, try proXSign at: - Test for creating a digital signature 

I found this blog article from April 11, 2013
http://romanmazi.blogspot.com/2013/04/dodatki-firefox-na-primeru-podpisne.html

I don't read Slovenian (too hard to copy'n'paste to Google translate from that blog for me to try and understand what he is explaining) - see if that might help you.

If you notice the screenshot of the install.rdf header, it shows a version number {1.2.21} of that extension - is your version the same version number?

Maybe a newer version is available somewhere?

Much appreciated, ed-meister, especially so for the ref from April 2013,

In a nutshell it says the following: install the add-on by hand as suggested here, bypassing the built-in version test.

It evidently worked - however, this is not the way to handle this kind of components, the entities, distributing the add-on, should update it for/to the latest FX available.

Re version: the one XPI I was talking about, is indeed older - 1.2.9. However, even 1.2.21 had to be installed by hand. The page with the ref to it

Ministry of Interior install proXSign page

says explicitely "does not work for FX 22 and higher". So there's outdated versions floating around - while Mozilla is raising the bar every six weeks or so;)

So, this is an extension that is signed with a certificate and Firefox doesn't have its root certificate installed.

An alternative is to remove the META-INF folder that stores the certificate data.
That should make Firefox install the nproXSign11.dll file (maxVersion of the extension is 40.*)