Unknown certificate issuer errors

Hi,
I just noticed that addons in both my Firefox portable instances didn't update for more than a month whereas the same addons in my non portable Firefox installation updated correctly.
So I tried to manually update the addons in my portable Firefox but I got errors: "There was an errror downloading <extension_name>" for each not up-to-date addon.
In the "Tools > Web Developer > Browser Console", there were more details:
"addons.cdn.mozilla.net:443 uses an invalid security certificate.

The certificate is not trusted because the issuer certificate is unknown.
The server might not be sending the appropriate intermediate certificates.
An additional root certificate may need to be imported.

Error code: <a id="errorCode" title="SEC_ERROR_UNKNOWN_ISSUER">SEC_ERROR_UNKNOWN_ISSUER</a>"

I also get similar errors when I visit Amazon sites: images & css don't load in FF portable
"images-na.ssl-images-amazon.com:443 uses an invalid security certificate.

The certificate is not trusted because the issuer certificate is unknown.
The server might not be sending the appropriate intermediate certificates.
An additional root certificate may need to be imported.

Error code: <a id="errorCode" title="SEC_ERROR_UNKNOWN_ISSUER">SEC_ERROR_UNKNOWN_ISSUER</a>"

The sites load fine with my non-portable Firefox.

All instances (portable & non portable) use the same version: FF 48.0.

My antivirus is Eset NOD32 9 but all Firefox instances (portable & non portable) are configured the same way in it.

The only difference is that the non-portable is 32-bit whereas the portable ones are 64-bit but I don't know whether this is relevant to my problems. Can I trick the FF portable launcher to use the 32-bit instance (App/Firefox directory) instead of the 64-bit instance (App/Firefox64 directory)?

Thanks for your help