How can I know that my installed certificates (in Firefox), are (still) reliable?
You don't have a huge amount of choice. As I understand it, the root certificates that are automatically trusted by Firefox are actually embedded in Firefox's source code. It looks like you can choose to untrust them through the settings, but you can not actually remove one of the default root certs.
We all have to trust that Firefox has a rigorous and respectable process for including a CA as a trusted root. See Firefox's page on how they choose to include root certs. Since the root certs are embedded in the source code of the browser, the only way to remove one is for Firefox to push a security update, so we also have to trust them to do this in a timely manner if one of the trusted roots has a breach.
It's too dangerous to go alone... Take this:
This tool allows you to see if what you have for secure connections is indeed genuine (Fingerprint check). It is very important and I am glad you see the concern, many do not.
There is also some helpful web plugins that help. I cannot say they are reliable but they help.
Trust has to start somewhere. The Root CA Certs on your system are the start of that trust. There is no 100% way to validate them. You can try to obtain them from multiple locations to make sure that they are from the company they claim to be, but ultimately, installing them is a declaration that you trust them to say who you should trust.