That seems like an argument for maintaining a frozen repo of packages, not against containers.
I am not arguing against containers, I am arguing that nix is more reproducible. Containers can be used with nix and are useful in other ways.
an argument for maintaining a frozen repo of packages
This is essentially what nix does. In addition it verifies that the packages are identical to the packages specified in your flake.nix file.
You can only have a truly fully-reproducible build environment if you setup your toolchain to keep copies of every piece of external software so that you can do hermetic builds.
This is essentially what Nix does, except Nix verifies the external software is the same with checksums. It also does hermetic builds.
I think it depends on the website. There are some websites where chrome will work better either because chrome works better with certain libraries/technologies or because the developers put more time into optimizing for chrome.
On the other hand Firefox might have less bloat around telemetry that gives it an advantage too.