Missing favicon on extension pages in Firefox

Environment

• OS version: /
• Browser version: Firefox (works on Chromium based browsers)
• Extension version: ABP 3.12
• Last working version: ?

Steps to reproduce

1. Install extension
2. Open settings page
3. Change options.html in the URL to:

• first-run.html
• updates.html
• day1.html
• popup.html
• problem.html
• issue-reporter.html

Observed behavior

Favicon is missing from all extension pages, except the Options page.

Expected behavior

Favicon should be visible on all extension pages.

Further information

We already had a bug opened for favicon being missing from the first run page (https://gitlab.com/eyeo/adblockplus/abpui/adblockplusui/-/issues/227), it was closed as we moved most users to web based FRP.