The first alpha release in the new 5.0 series of the Tor Browser is now available from our extended downloads page as well as the distribution directory.

Tor Browser 5.0a1 is based on Firefox ESR 31.7.0, which features important security updates to Firefox.

In addition to including all of the fixes that were present in the 4.5.1 release, this alpha release also features some additional privacy defenses.

In particular, this release re-enables the automatic window resizing fingerprinting defense that first appeared in 4.5a4. This defense can be disabled by setting the about:config pref extensions.torbutton.resize_windows to false, but please first report any issues you encounter on the feature's trac ticket.

This release also introduces a new defense against various forms of performance fingerprinting and time-based side channel attacks. A handful of new attacks have been published recently that take advantage of Javascript's high-performance timers to determine hardware performance, perform keystroke fingerprinting, extract history information, and even steal sensitive data from memory. Because this defense reduces the resolution of time available to Javascript to 100 milliseconds for all time sources, and to 250 milliseconds for keypress event timestamps, we are especially interested in hearing any reports about issues with HTML5 video, animation, or game sites. Hopefully you will have as much fun testing this defense as we will!

Here is the complete list of changes since Tor Browser 4.5: