Thunderbird failing to find the signing certificate specified in my Mail & Newsgroup Account Settings

I managed to import my s/mime certificates following this, but since then, whenever I go to digitally sign my email, I keep getting this error. My certificate expires almost a year from now, and I haven’t moved my certificate after importing it, any idea how to solve this issue?
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You can try to use a portable version of Thunderbird similar to this topic:
Preinstalled Firefox can´t load PFX certificate but portable one can

Another possible workaround is to downgrade the crypto-policy:
Changes/StrongCryptoSettings2 - Fedora Project Wiki

What’s the portable version of Thunderbird? I installed it from the discover software repo as KMail was preinstalled in my desktop at start.

It’s distributed as an archive on the official sites:

Nope, even Thunderbird version 99.0b2 failed

You may use a certificate with SHA-1.
This can help:
https://blog.thunderbird.net/2023/10/thunderbird-115-and-signatures-using-the-obsolete-sha-1-algorithm/

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Another problem might be that the certificate authority’s certificate is not installed.