Digital certificate does not appear on certificates list to sign a file in LibreOffice

Linux ubuntu 18.04 - Libreoffice 6.0

Firefox has changed the way they made key for digital signature since 2018. So the key are not accepted by libreoffice any more. Fortunately, I can use old key from old version of firefox or use palemoon (another form of firefox based on old firefox - https://www.palemoon.org/) to make my key.
Why Libreoffice don’t upgrade to accept the new key ?

solved - I have upgraded to ubuntu 20.04 / libreoffice 6.4.3.2 - working OK

@FredK: I reopened your question because you closed it wit reason “Duplicate question” whereas it looks rather an adequate answer has been provided. If there is really a duplicate to this one, please provide a link to it before closing.

Well, I am very new here, it was the first time I close a question; the “reason” has been edited already to “the question is answered, right answer was accepted by Fredk , close date 2020-06-14 17:57:19.655559”.

Thanks. I’ll check your answer as soon as the deadline allows it. This will increment your “karma”.