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I think that 80% of the onion sites I'm trying to reach are most of the time (if not always) unavailable. It tells me that
Firefox can't estabilish a connection to the server at *****.onion
The site could be temporarily unavailable or too busy. Try again in a few moments. If you are unable to load any pages, check your computer's network connection. If your computer or network is protected by a firewall or proxy, make sure that Tor Browser is permitted to access the Web.
I'm running TBB 4.0.1 for x64 Linux and I don't use firewalls or proxy servers.
Why does it happen and how can I fix this issue?