Bug #4755: Return client window coordinates for mouse event screenX/Y (for dragend, 0,0 is returned).
Bug #8312: Remove "This plugin is disabled" barrier.

We do not want to encourage our users to enable plugins at this point.

The barrier also causes bad UX for HTML5 video fallback on YouTube.
Bug #3875: Use Optimistic Data SOCKS variant.

This patch alters Firefox's SOCKS handshake to preemptively send data before
it is actually connected. This allows us to save a round trip during
connection setup.

See:
https://gitweb.torproject.org/torspec.git/blob/HEAD:/proposals/181-optimistic-data-client.txt
TB3: Tor Browser's official .mozconfigs.
TB4: Tor Browser's Firefox preference overrides.

This hack directly includes our preference changes in omni.ja.
Bug #3688: Build determinism fix: Disable library timestamping.

This timestamp prevents deterministic builds, and is set in a way that
libfaketime can't correct.
Bug #6253: Add canvas image extraction prompt.