As far as I can tell, mozlz4 is just a special file extension for a standard lz4 compressed archive in the same way that .xpi is a special file extension for a deflate (.zip) archive, just so that mozilla can do a quick search and load any files with that extension. Being LZ4 compressed theoretically means that the file is faster to decompress vs competing algorithms.
LZ4 is in no way Mozilla specific, it just hasn't been added to tools like 7zip that have a hard enough time keeping up with every new archive format coming out while also trying to improve the speed and stability of their product. Not to mention 7zip's focus is it's own archive format.
There's a fork of 7zip with additional compression formats included, but I've never used it and can't attest to it's stability vs standard 7zip.