Can Firebase functions.config() still be used in first-generation functions?

Yes, you can use functions.config() when using those dependency versions. .config() is supported for first-generation Cloud Functions, but please note that the generation of Functions matters more than the versions. So, in short, the use of functions.config() it is determined by the Cloud Functions generation, not the npm package version. Also, Firebase maintains backward compatibility for 1st-gen features. However, this practice is called legacy, and Firbease recommends migrating to the 2nd-gen Functuons.