Try the Firefox version from the official Mozilla server if you currently use a version from the repositories of your Linux distribution.

I tried it, and the import option is still not there.

It looks like the import option was removed:

https://bugzilla.mozilla.org/show_bug.cgi?id=1064402

But I don't know how to do it now when one server certificate is needed. I'm trying to configure Firefox to use my Spanish ID card, and it requires a server certificate.

What kind of certificate do you want to import?

The server tab seems to be storing exceptions. Certificates are stored in the authorities (intermediate certificates) and people tabs.

I have both an "authority" certificate and a "server" certificate. The instructions that I have for Firefox say that the server certificate has to be stored in the "server" tab, and the screenshots show that it has an import option. I remember that it was there as I did this before, but now it seems it has been deprecated, according to the bug report above. So I'm not sure if it is needed any more or not.

Is this an intermediate certificate that chains to a built-in root certificate or a (self-signed) certificate for a specific server that works on its own? An intermediate certificate can be imported under the authorities tab. Other certificates can be imported under "your certificates" (website) or "people". You can also drag a certificate in a Firefox window or use "File > Open File" to import a certificate.