Edge is developed by Microsoft, so we have no control over what it does or doesn't do. Edge is used by other parts of Windows (such as explorer) which may be re-creating the cache. Firefox's cache is entirely separate and doesn't touch edge in any way

Thanks Tyler! Have there been other questions about this ? Do you have any suggestions about stopping this? Can MS be tracking my internet browsing? I always use private browsing.

There's nothing to stop if you are using Firefox. Private browsing only keeps your history from being saved permanently on your computer. Edge is going to do it's own thing, I wouldn't worry about it. Just keep using Firefox

I have this same problem plus MS IE shows a ton of cookies and temp files even when I clear all temp and cookie files before opening Firefox. Can Edge and IE be safely deleted just so that everything stays private? Even when I use TOR both load up on cookies and temp files. Being a privacy freak, are any of these browsing files available to MS to sell?

It is no comfort that Edge, supposedly a browser, continues to operate and gather who knows what information that MS will use if they are collecting it. The interesting question is whether Firefox actually uses Edge to gather information for their own use. BTW, this does not happen in Opera.