Thunderbird 60.3.0 - Option 'open in conversation' is not available

Good evening folks,
I've setup a server with GUI and installed Thunderbird 60.3.0 from rhel-7-server-optional-rpms.

I've grouped my mails by conversation. But when I expand a conversation, right click on some mail (not the first one) there is no option 'open in conversation' in the context menu.

Now, I'm unsure if this is a Bug or if this feature is just not available in the package build for RHEL 7.

Does anyone have this option or could confirm that it should be there but is missing?

Best regards,
Joerg