8-month-old Tor Browser offers to "Reset Tor Browser", removes extensions

I started an old copy of Tor Browser 4.0 I had in a directory. From file times it looks like it was last run on 2014-10-21, 8 months ago (today is 2015-06-24). When I started it, the about:tor page showed a banner at the bottom It looks like you haven't started Tor Browser in a while. Do you want to clean it up for a fresh, like-new experience? And by the way, welcome back! with a button that says "Reset Tor Browser..."

When I click on the button, the browser restarts and gives me the plain Firefox start page, with the Firefox logo and everything. It looks like all the extensions are gone (HTTPS-Everywhere, NoScript, Torbutton, Tor Launcher). check.torproject.org says the browser is using Tor. I also got a banner offering to update to 5.0a2.

I was trying the instructions from [[doc/TorBrowser/Hacking#UsinganExistingTorProcess]] so I had run

export TOR_SOCKS_PORT=9050
export TOR_SKIP_LAUNCH=1
./start-tor-browser

I don't know if that had anything to do with it.