I’m on Linux as well, so that’s probably not it. But I just checked the release notes, and it does say:

This feature is part of a progressive roll out.

What is a progressive roll out? Certain new Firefox features are released gradually. This means some users will see the feature before everyone does. This approach helps to get early feedback to catch bugs and improve behavior quickly, meaning more Firefox users overall have a better experience.

So hopefully you’ll get it soon!