Help page is broken, when Tab Center add-on is enabled

[ciastek]

• I've searched for duplicates – also closed issues
• I've looked at the change log
• I've provided the below details:

VimFx version: 0.20.3
Tab Center version: 1.26.0
Firefox version: 51.0a2
OS: OS X El Capitan (10.11.6)
Keyboard language: Polish Pro
Example URL (if any): about:

This is … (pick one!)

• A bug report
  • I've filled in steps to reproduce below
• A feature request
  • I've given a detailed example below
• Somehting else

This bug is about … (pick one!)

• A missing hint marker on a specific page
• VimFx not recognizing a text input on a specific page
• Something else

Steps to reproduce:

1. Have a Tab Center add-on enabled. (Install Test Pilot add-on and enable Tab Center there)
2. Press ? to show help page.

Expected result: There should be help page displayed. (Screenshot shows the result with Tab Center disabled)

Actual result: Only small white square is displayed instead. (Screenshot shows the result with Tab Center enabled)