Signing in from the welcome page will quit Firefox if it's the only tab open

[rfk]

STR:

• Install the lockbox addon
• Click the lockbox toolbar icon to open the "Welcome to Lockbox" tab (/firstrun/index.html)
• Close all your other tabs
• Click "Sign in to your Firefox Account"

Expected result:

• I get the OAuth popup window and can sign in to Lockbox.

Actual result:

• The tab with the welcome page closes itself
• Firefox quits because there are no more tabs open
• I freak out for a few moments, thinking that I somehow just crashed Firefox using a webextension, before realizing what had happened :-)

[m8ttyB]

I can reproduce this on 0.1.7 alpha1.

[linuxwolf]

Related to #427