Merge list_toolchain_updates-firefox-* commands into one

Running list_toolchain_updates-firefox-linux, list_toolchain_updates-firefox-macos and list_toolchain_updates-firefox-windows in 3 separate build steps means generating and extracting the same firefox source tarball 3 times, which is taking some time.

We should try to merge them into one single build to make that faster.

The reason for doing it in 3 builds is to access config options using separate targets for each platform, but we might still be able to do that within a single build.