TBB unconditionally logs all Firefox output to disk
Previously, Tor Browser Bundle's startup script would check whether its stdout and stderr are pointed at a terminal, and close them otherwise (unless the user is explicitly running TBB in debug mode). This allowed power users to easily view TBB's error messages in a terminal, while ensuring that novice users who start TBB from a GUI file manager would not accidentally leave evidence that they had used TBB in ~/.xsession-errors
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Now, the startup script unconditionally redirects Firefox's output into a disk file, and according to [ticket:11751#comment:6], that disk file will contain sensitive information (“it often includes full URIs logged from HTTPSEverywhere”).