Element picker not working on support.mozilla.org #512

Prerequisites

• I verified that this is not a filter issue
  • Filter issues MUST be reported at filter issue tracker
• This is not a support issue or a question
  • Support issues and questions are handled at /r/uBlockOrigin
• I performed a cursory search of the issue tracker to avoid opening a duplicate issue
  • Your issue may already be reported.
• I tried to reproduce the issue when...
  • uBlock Origin is the only extension
  • uBlock Origin with default lists/settings
  • using a new, unmodified browser profile
• I am running the latest version of uBlock Origin
• I checked the documentation to understand that the issue I report is not a normal behavior

Description

The element picker doesn't work on Mozilla's support website.

A specific URL where the issue occurs

On some rare websites, but the only one I have in head at this moment is the Mozilla's support website.
https://support.mozilla.org/
You can reproduce the bug on every page of this domain as well.

Steps to Reproduce

1. Go to "https://support.mozilla.org/".
2. Left click to the "uBlock Origin" icon to open its menu.
3. Click to the element picker icon in order to hide, for example, the black sticky header.
4. See that you are not entered in picker mode. The element zapper is not working either.

Expected behavior:

The element picker should works on this site as it works everywhere else.

Actual behavior:

The element picker doesn't work at all on this site, nor the element zapper.

Your environment

• uBlock Origin version: 1.18.16
• Browser Name and version: Firefox 66.0.2 (64 bits)
• Operating System and version: Issue occuring on both Windows 7 and 10 (both in 64 bits)