Well for Firefox, the one getting updated is the native rpm version which is part of the standard Silverblue install while the one already updated is the flatpak version. The native version is just called 'Firefox' while the one from flatpak is called 'Firefox Web Browser' if I remember correctly. I have no idea why signal is showing up there. Maybe it is a bug.

Also next time a system update is shown in GNOME software, check using rpm-ostree status to see if any updated image is staged. If yes, then you don't have to bother with gnome software - when you shutdown or reboot, the update will automatically be applied.

Those might be flatpak "refreshes", which show up as "updating to the same version". As described by a flatpak maintainer, sometimes an app or runtime gets updated without changing the user-facing version number. I assume that's what you're seeing here.

You are using Firefox installed through flatpak, but the base image also contains Firefox. Even if you remove it with rpm-ostree override remove it will continue be showed in the list of updates. I'm not sure for GNOME Software because I don't use it. And what is for Signal, maybe it's a GNOME Software issue.

flatpaks would be my first goto

Distributing software is not instantaneous. Assuming that Mozilla has already sent the update to flathub, it will take some time before it's validated and available for download.

If instead of flatpak you had used native packages, you would be in the same situation, as fedora's update system keeps updates in testing until enough people say it's fine.

If you wanted to get the update as soon as possible, you would have to download the prebuilt binary from Mozilla, but then you would have to update manually and everything.

Just be patient for a few days.