libmozsandbox.so error on Debian jessie amd64 after upgrading to 7.0a3

This may be related to #21907 (moved).

Tor Browser downloaded some update yesterday. Today I started it and it showed a progress bar and a message like "Please wait a few moments while we apply the updates" and then the browser didn't start. Trying two start it manually in the command line causes:

$ ./start-tor-browser.desktop -v
Launching './Browser/start-tor-browser --detach -v'...
XPCOMGlueLoad error for file /home/david/tor-browser_en-US/Browser/libxul.so:
libmozsandbox.so: cannot open shared object file: No such file or directory
Couldn't load XPCOM.

I would have expected that the update was for 7.0a3, released yesterday. Bug I am not sure what exact version it thinks it is, because I see 7.0a1, 7.0a2, and 7.0a3 in various places when I grep:

$ grep -rF '7.0a'
...
Browser/TorBrowser/Docs/sources/versions.moz-backup:TORBROWSER_VERSION=7.0a1
Browser/TorBrowser/Docs/sources/versions:TORBROWSER_VERSION=7.0a2
...
Browser/TorBrowser/Data/Browser/profile.default/prefs.js:user_pref("browser.startup.homepage_override.torbrowser.version", "7.0a2");
Browser/TorBrowser/Data/Browser/profile.default/prefs.js:user_pref("extensions.lastTorBrowserVersion", "7.0a2");
Browser/TorBrowser/Data/Browser/profile.default/prefs.js:user_pref("extensions.torbutton.lastBrowserVersion", "7.0a2");
Browser/TorBrowser/UpdateInfo/updates/0/update.version:7.0a3
...