It’s not just about features, there is a list of security fixes.

"Support for credit card autofill has been extended to users running Firefox in the IT, ES ,AT, BE, and PL locales.

macOS users can now control the tabability of controls and links via about:preferences.
Screenshot of new macOS tabability option in about:preferences

To avoid undesirable outcomes on sites which specify their own behavior when pressing shift+right-click, Firefox now has a dom.event.contextmenu.shift_suppresses_event preference to prevent the context menu from appearing."

Other than that yeah

As a developer Nested CSS support is a huge QoL improvement. I enabled it manually previously to work on a site assuming that it would be fully supported by time the site launched, so I’m certainly glad they stuck to their roadmap on that.

Sorry, but why are you trying to use a flatpak version of Firefox? You can visit ftp.mozilla.org and download the Linux binaries easily!!!