nowhere in solus help center are flatpak installation instructions. you probably mean flatpak's own rec for installing on solus?
but if you installed right you should be good to go. maybe ignore mozilla right now. and update flatpak. it should have pulled in org.freedesktop.Platsform
as it installed itself I think.
If not install it for kicks. flatpak install org.freedesktop.Platsform
orrm
flatpak and try again. find some internet instructions. it should be a 2 part process: adding/installing the flathub repo then activating it if I recall right.
that's a way to go about it.
another way is to install something else and see if you get the TLS error.
if you do you got a certificate error possibly it might be an install/reinstall certificates problem.
a variety of online solutions exist for this so trial and error. hope I gave you someplace to start troubleshooting. good luck.