Firefox: Prepare for non-persistent background page #2969

Description

Admittedly, I am a bit late at this.

See:

• https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/Background_scripts#convert_to_non-persistent
• https://discourse.mozilla.org/t/is-non-persistent-background-script-mandatory-for-android/123839

To do:

• Verify and fix as needed that auxiliary pages are behaving properly when the uBO background page is terminated
• Evaluate how port closing affects content scripts -- this might be the most problematic aspect of being non-persistent
  • This means the generic cosmetic filtering surveyor may be interrupted even though there still could be matching generic cosmetic filters to come
• Optimize timing to generate a selfie, which will be even more key to quickly load back uBO's process into memory
• Evaluate using storage.session to further speed up load sequence