check 2603 browser.helperApps.deleteTempFileOnExit

```js
/* 2603: remove temp files opened with an external application
 * [1] https://bugzilla.mozilla.org/302433 ***/
user_pref("browser.helperApps.deleteTempFileOnExit", true);
```

I think this also requires `browser.download.start_downloads_in_tmp_dir`

also see https://gitlab.torproject.org/tpo/applications/tor-browser/-/issues/42147