Is it safe to simply rename a .jfif file to .jpg?

Yes.

JPEG is not a file format - it's a compression method. It doesn't specify how the compressed data should be stored in a file. There are two file formats that fill in this gap: the original JFIF, and the newer Exif¹. They are very similar, mostly compatible and can even be mixed in a single file. Both use the .jpg extension.

Your .jfif file is a JPEG-compressed image stored in a JFIF file format. It's a valid content for a .jpg file.

¹ Technically, TIFF also supports JPEG data, but I don't think I've ever seen such file and it's certainly not a common choice for saving JPEGs.