Yes, PATH_TO_TORBROWSER is the very same (full) path to the Tor Browser directory for both configurations mentioned above.

A new Tor Browser instance via the firefox.exe (second config mentioned above) results in creating a new tor connection which conflicts with the already up and running tor connection. So I don't think that this is a solution.

As for the first configuration mentioned above, when executing it in a command prompt the expected behaviour is observed --> a new tab with the correct link is openend in the already running tor browser. The very same command, for some reason, doesn't work in JDownloader and keeps openening a new tab in the default browser instead of the tor browser.

Are there anywhere logs which I could have a look at? If so, is the log verbosity level configurable?