Actually, in the case of a web browser, Flatpak weakens both Firefox’s and Chromium’s internal sandboxing, possibly allowing for breaking of cross-site or site-host boundaries. Firefox is even weaker then Chromium as a Flatpak because it can’t use the zypak fork server. Both are weakened, best to avoid.
For basically any other app, Flatpak can be beneficial as a sandbox.
Basically, don’t sandbox browsers because its like wearing 2 condoms. The only sandboxing tool I know that doesn’t interfere with the browser’s sandbox (and also doesnt allow for the possibility of privilege escalation, like Firejail) is Bubblejail
PS: Since you mentioned you are on Fedora, Bubblejail is offered through this COPR repo from the Secureblue team. It provides a sandbox without interfering with the browser’s sandbox. It comes with profiles for Firefox and Chromium. Only issue ive experienced is that the sandbox works, aka it means I can’t access files from my home directory unless explicitly given permission to a folder.
It also has a Flatpak release, which I prefer for the ability to restrict permissions like internet.