Upgrade Apple TV app on Firestick.

Apple TV will not bookmark a movie after pausing in Firestick.

The Apple TV app is intended to mark partially watched content as such, and allow viewers to continue where they left off. This should be reflected for episodes of TV shows and for movies, both in their own listing, and in Up Next. This watch progress information is stored as an account property, so verify that you are signed in correctly.

If it doesn’t show that, then the Apple TV app is not behaving as it should. Consider checking for newer device system software, and install that if available. Consider deleting and re-installing the Apple TV app. Else contact Apple Support for further assistance.

Nor will it access Extras in purchased movies.

This is intentional, and exclusive to Apple devices.

If you can't view your iTunes Extras - Apple Support

“iTunes Extras aren't available in the Apple TV app on smart TVs and [third party] streaming devices.”

Is there an upgrade?

I would consider the Apple TV box an upgrade, but I doubt that matches what you meant.

For updates to the Apple TV app: Your device should have an automatic app update function (all apps), where it periodically checks for newer versions.

If you want Apple to know about your thoughts for improvement, then please leave feedback: Feedback - Apple TV App - Apple