Tor creates temp files in /tmp when downloading files, this is obliviously a security issue. How can i prevent Tor from using /tmp ?
Well, Tor does not create it - it's daemon is just a network service. The browser - it's a separate binary, so - yes, you're right: it's a privacy leak indeed. To mitigate it use either a VM routed through Tor with a regular browser or use QubesOS with a disposable VM for that kind of tasks. If you have to use Tor browser in a routed VM - make sure that your Tor will use your another Tor instance to access the Tor network
For convenience you could use Firejail.
3.1 Private Mode
Private mode is a quick way to hide all the files in your home directory from sandboxed programs. Enable it using --private command line option:
$ firejail –private firefox
Firejail mounts a temporary tmpfs filesystem on top of /home/user directory. Any files created in this directory will be deleted when you close the sandbox. You can also use an existing directory as home for your sandbox, allowing you to have a persistent home:
$ firejail –private=~/my_private_dir firefox
If you already have an instance of Firefox running you can do this to test:
firejail --private firefox -no-remote
For the Tor browser you just need to adjust the executable path accordingly.