Bug 1931609

This is a 2 years old upstream/project bug https://gitlab.gnome.org/GNOME/seahorse/-/issues/205
Ubuntu reference https://bugs.launchpad.net/ubuntu/+source/seahorse/+bug/1771880
I became aware of it about an hour ago when I tried to import my S/MIME cert.
If Fedora people have/know some workaround I'll be happy to try it.

GPA (https://gnupg.org/software/gpa) hasn't changed since October 2018. The only actively maintained project is Kleopatra (https://dev.gnupg.org/tag/kleopatra).
I do hope GNOME/Seahorse will catch up. Meanwhile, I think I will replace Seahorse with Kleopatra on my system until we get some good news/updates here.
See also https://medium.com/beingcoders/easy-way-to-convert-pfx-to-crt-key-files-3a75bb283dab as a possible workaround.

Quick update: the good news is that Mozilla Thunderbird (one of my favorite MUA) can handle .pfx without flexing a muscle.
Yes, that's right. If you're looking to import your S/MIME certificate in Mozilla Thunderbird here is how:
- open Account Settings;
- under [each] account;
- there are several configuration items/sections: Server Settings, Copies & Folders, Composition & Addressing, Junk Settings, Synchronization & Storage, End-To-End Encryption, Return Receipts;
- select End-To-End Encryption section: here is where you can configure either OpenPGP or S/MIME;
- press button Manage S/MIME Certificates which will open the Certificate Manager dialog window;
- pay attention to the sections on that dialog: Your Certificates, Authentication Decisions, People, Servers, Authorities;
- select Your Certificates section then press Import... button (on that section);
- on the open/select file dialog that will pop up next, there is a file type filter (bottom right corner) which contains "PKCS12 Files";
- the rest is pretty obvious: using that filter, you'll see and select your cert.pfx file and you'll be prompted for password to unlock it;
Of course, the S/MIME certificate must match the account, be valid, issued by a recognized authority. Good luck!

This message is a reminder that Fedora 33 is nearing its end of life.
Fedora will stop maintaining and issuing updates for Fedora 33 on 2021-11-30.
It is Fedora's policy to close all bug reports from releases that are no longer
maintained. At that time this bug will be closed as EOL if it remains open with a
Fedora 'version' of '33'.

Package Maintainer: If you wish for this bug to remain open because you
plan to fix it in a currently maintained version, simply change the 'version' 
to a later Fedora version.

Thank you for reporting this issue and we are sorry that we were not 
able to fix it before Fedora 33 is end of life. If you would still like 
to see this bug fixed and are able to reproduce it against a later version 
of Fedora, you are encouraged  change the 'version' to a later Fedora 
version prior this bug is closed as described in the policy above.

Although we aim to fix as many bugs as possible during every release's 
lifetime, sometimes those efforts are overtaken by events. Often a 
more recent Fedora release includes newer upstream software that fixes 
bugs or makes them obsolete.

This message is a reminder that Fedora 33 is nearing its end of life.
Fedora will stop maintaining and issuing updates for Fedora 33 on 2021-11-30.
It is Fedora's policy to close all bug reports from releases that are no longer
maintained. At that time this bug will be closed as EOL if it remains open with a
Fedora 'version' of '33'.

Package Maintainer: If you wish for this bug to remain open because you
plan to fix it in a currently maintained version, simply change the 'version' 
to a later Fedora version.

Thank you for reporting this issue and we are sorry that we were not 
able to fix it before Fedora 33 is end of life. If you would still like 
to see this bug fixed and are able to reproduce it against a later version 
of Fedora, you are encouraged  change the 'version' to a later Fedora 
version prior this bug is closed as described in the policy above.

Although we aim to fix as many bugs as possible during every release's 
lifetime, sometimes those efforts are overtaken by events. Often a 
more recent Fedora release includes newer upstream software that fixes 
bugs or makes them obsolete.

Fedora 33 changed to end-of-life (EOL) status on 2021-11-30. Fedora 33 is
no longer maintained, which means that it will not receive any further
security or bug fix updates. As a result we are closing this bug.

If you can reproduce this bug against a currently maintained version of
Fedora please feel free to reopen this bug against that version. If you
are unable to reopen this bug, please file a new report against the
current release. If you experience problems, please add a comment to this
bug.

Thank you for reporting this bug and we are sorry it could not be fixed.