This is explained in bug 1872395.

Please remove cache2 folder.

Microsoft Defender has been the only scanner reporting this alleged Trojan in the Cache folder for a while now. Check for definitions updates if any in your scanner.

You are not infected if this was only found in the Cache folder as it is harmless if left alone there. It will either get overwritten over time as Cache gets used or deleted if you clear the Cache. https://support.mozilla.org/en-US/kb/how-clear-firefox-cache

This old KB article was for older versions of Firefox in mind with the settings, however still relevant. https://support.mozilla.org/en-US/kb/Firefox%20cache%20file%20was%20infected%20with%20a%20virus

Note that this could be a problem with creating the backup while Firefox is running by using a special ShadowCopy technique that takes a snapshot of the current state, you can never be sure that restoring such a backup would work properly as the disk cache is updated extensively and the cache index files may only be written to the hard drive when you close Firefox, so there is likely a mismatch/inconsistency.

There is no real need to backup data stored in the secondary profile location in "AppData\Local" as only the disk cache and other temporary is stored there. The main profile folder with your personal data like bookmarks and logins is stored in "AppData\Roaming".

• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data