If it exists, it's likely buried in about:config, which is guarded by a warning telling you to go away. And eventually it will be removed because no one[1] used it, which proves that the designers were right.
[1] Except for some weirdos who disabled telemetry. They're probably so paranoid they believe Mozilla would abuse the studies feature to promote a TV series or something like that.
I guess there's an option somewhere to set opacity to 100% but I'm not ready to spend 30 minutes to hunt it down.