Extension icon re-appears in Firefox

[thekevinbrown]

Reproduction Steps

• Run python kango-dir/kango.py create . in a new project directory
• Use the project name of Test
• Run python kango-dir/kango.py build .
• Load the .xpi into Firefox
• Right click and select "Remove from Toolbar"

• Reboot the computer
• Open Firefox again

Expected Result

Beaker icon and extension should stay hidden from the toolbar

Actual Result

The beaker icon and extension reappear on the toolbar

OS: OSX 10.11.5
Firefox: 47.0.1

We're seeing this behaviour reported by users in our extension here: toolkit-for-ynab/toolkit-for-ynab#308

It's also worth noting that other non-kango extensions don't seem to act this way.