Here is the similar topic mentioned by @DavidR: Firefox doesn’t recognize the certificate issuer if HTTPS scanning is on (with repro).
In summary:
- It was an Avast bug in Firefox, now it seems to come up in Chrome.
- Indeed Avast needs to inject its certificate to be able to scan HTTPS websites, but this injection is expected to be invisible to you (it’s called “transparent” HTTPS scanning). If it’s visible, it’s a bug.