Drop a JFIF above, get a JPG back instantly. No recompression, no quality change: the bytes come back exactly as they went in, renamed.
A JFIF already is a JPEG. The name stands for JPEG File Interchange Format, the standard that plain JPG files have followed since 1992. Windows occasionally saves a downloaded image as .jfif when a website labels it loosely, and then half your software pretends not to recognise a format it has read for thirty years.
Because the formats are identical, this tool does not convert anything. It hands the file back with a .jpg extension, byte for byte, without decoding or re-encoding anything, and nothing is lost in the process. Renaming the file yourself does the same job; this page just does batches, and explains why the problem exists.
The DIY fix: show file extensions in Explorer, rename .jfif to .jpg, done. The image data never needed changing.
Stopping it recurring is harder: the extension is chosen at download time, and on Windows it comes from the registry entry for image/jpeg, which on a good many machines reads .jfif.
Batches process at once, with a ZIP at the end.