I am running a Linux-PC with Firefox last version. Is it normal the .cache/mozilla/firefox/ to contain files of the type 'application/octet-stream'?
It's entirely normal - there is no reason to exclude files based on their type.
I can see such files in my own cache. I'm using Nirsoft's MZCacheView in Windows.
application/octet-stream is nothing more or less than "this is some uncategorised data"
It could be an image, a video or just some gzip compressed webpage data.
Not all web servers properly check and send the correct file type for data and instead leave it up to your browser to download the file, check the data for type information and then display as necessary.
It is not a sign of anything out of the ordinary, nor is there anything actually wrong.