You can contact MS if you have a problem with their software, those are likely false positives and you can ignore them. This is about data stored in the Firefox disk cache (about:config). You can exclude the cache2 folder in Windows Defender to prevent such false positive notifications.

• https://support.mozilla.org/en-US/kb/Firefox+cache+file+was+infected+with+a+virus

Seems to be an issue with Microsoft Defender when it is the only scanner that has been having this issue in the last weeks now. Make sure you check for updates each day to see if they fixed it.