As there is no screenshot error or what site is causing this or what steps anyone is able to replicate the problem this would have to be a computer shop to diagnosis the problem.

One specific cache entry always contains a trojan virus everytime I run an anti virus check. I can't say where this cache comes from as it appears even if I don't open a single new website. I delete it but it appears everytime I use the Browser again, even multiple times during one session. It is always the same entry code. Is there a way to block a specific cache entry or at least to discern where it originates?

Either it is making a false positive or some site you are going to keeps adding something to the Firefox Cache that your antivirus thinks is a trojan. A trojan is the Cache is harmless if left alone to be over written or if you delete the Cache.

An old article on this is https://support.mozilla.org/en-US/kb/Firefox%20cache%20file%20was%20infected%20with%20a%20virus though the steps in Settings have changed https://support.mozilla.org/en-US/kb/how-clear-firefox-cache

How much information do you have about the cached file? You can search for its name on the following page:

Type or paste about:cache in the address bar and press Enter to load it.

Under disk, click "List cache entries"

Exercise patience because usually there is a tons of metadata to list out.

Use Ctrl+F to search for the infected file name, and it should have the full URL back to the site where you got it.

Now, if that's not a site you visited intentionally, you might wonder if it's related to one of your add-ons, so further investigation might be needed.