I guess I'll try to formulate a full answer to this.
Your privilege seperation is security theatre.
Running Tor Browser as another user inside of the same X environment means the Tor Browser, regardless of an UID it is run under, can influence anything you can through your desktop, as an example see mjg59's
Circumventing Ubuntu Snap confinement.
Your packet filtering is misguided.
You said "for example the "tor_browser" has only the right to use the http and https ports". The Tor Browser only uses ports 9150 and 9151. 9150 is the SOCKS where connections to arbitrary ends-points can be negotiated. Restricting tor "tor_browser" user ports 80 and 443 only restricts the selection of guards that the tor instance running as "tor_browser" can make.
What should I do?
If you want to create a tails-like setup where a packet filter enforces tor-only traffic, then you should instead run tor as a system daemon under a low privileged user and allow it and only it to send traffic out to the internet, then configure your Tor Browser running as your user to connect to the system daemon. You can do this by following this guide.
But I'd like better security still
You might consider using some of the modern sandboxing mechanisms built into the linux kernel. Create a new network namespace so the process only sees the local loopback device, create a read-only filesystem and mount a tmpfs with your copy of tor browser copied into it, use seccomp sandboxing (at least blacklisting if not whitelisting) to reduce the damage an exploit might do and any privilege escalation or lateral movement it might perform. You could also consider, along with these steps, using something like Xephyr or xpra to create a sandboxed X environment within your normal X environment. This would stop Tor Browser from being able to read your keystrokes to other windows or inserting its own keystrokes into other X windows. This is how Subgraph approaches the problem.
The alternative is to use a hypervisor and run entirely distinct operating systems to achieve the same effects at slightly more cost for more clean and distinct isolation, as per Qubes OS or Whonix.
