Does it still happen with the flatpak version?

Now that you mention it, I am sorry I forgot to say that I am using Firefox ESR from:

https://ppa.launchpadcontent.net/mozillateam/ppa/ubuntu/

For example, the version I am using right now is "115.7.0esr (64-bit)".

The reason why I switched to the PPA is that I found the constant nagging from the default Snap package very annoying:

 Pending update of "firefox" snap
 Close the app to avoid disruptions (4 days left)

You are probably asking about the Flatpak version because Snaps have other issues too. For example, the Snap Firefox does not work on x2go remote desktops, and the cause is not clear.

I don't know much about Flatpak. Is it worth testing it? After all, I am already using an official source from Mozilla, just a different one.

Close the app to avoid disruptions (4 days left)

Try not to delay security updates for over week if you can help it.

The snap and flatpak versions will be more consistent across different distros.

You should use the latest 122 version unless you have a specific reason to use ESR.

Try not to delay security updates for over week if you can help it.

As far as I understand the ESR releases, I am not delaying security updates. ESR version 115.7.0esr is the current ESR version and should be up to date with regard to security fixes. Like I said, the PPA is an official source from Mozilla too.

The snap and flatpak versions will be more consistent across different distros.

But they bring their own problems, like I said for the Snap version. It is not only what I have experienced myself, it is widely known that Snap packages have restrictions and issues which cause pain all over the place.

You should use the latest 122 version unless you have a specific reason to use ESR.

As far as I understand, you do not need a specific reason to use ESR, as they are supported and should be more stable than the normal releases.

The main reason why I am using ESR though is that the PPA tends to conflict with the normal repository. The official instructions involve using APT pinning to prefer the PPA version over the default one (which installs the Snap variant). There is no ESR version in Ubuntu, so by choosing the ESR version there is no conflict to avoid.

All this has not a lot to do with the question though: I would expect the Enter key to work fine with the ESR version from Mozilla's PPA. I haven't recorded it actively, but I think this issue has been going on for more than 6 months, so it should have affected more than 1 ESR release.