I haven't used Arch in a decade, but as unstable as Arch is, I don't think Arch doesn't have dependencies poorly defined like that.
Say, firefox-1 does not depend on libssl-1, but now you upgrade it to firefox-2, you won't succeed if you successfully downloaded firefox-2, but failed to download libssl-1, because pacman shall fail, while saying the reason being failed to download all of its dependencies.
If you start with a system with both firefox-1 and libssl-1 installed, upgrading firefox-1 to firefox-2 sure would have no problem, because its dependencies are already fulfilled.
If your system is breaking, it's probably due to some other issue, but it could not be pacman's.