Fix #6888: Fixes __firefox__ overrides to not override themselves

[Brandon-T]

Summary of Changes

• Fixes firefox overrides to not override themselves
• Recursive overriding can cause issues on twitch which overrides XMLHttpRequest.prototype.send and tries to do its own security on top of our own.

This pull request fixes #6888

Submitter Checklist:

• Unit Tests are updated to cover new or changed functionality
• User-facing strings use NSLocalizableString()
• New or updated UI has been tested across:
  • Light & dark mode
  • Different size classes (iPhone, landscape, iPad)
  • Different dynamic type sizes

Test Plan:

1. Visit twitch.tv
2. Refresh the page
3. Twitch should NOT freeze! The page should refresh and load just fine.

Reviewer Checklist:

• Issues include necessary QA labels:
  • QA/(Yes|No)
  • bug / enhancement
• Necessary security reviews have taken place.
• Adequate unit test coverage exists to prevent regressions.
• Adequate test plan exists for QA to validate (if applicable).
• Issue and pull request is assigned to a milestone (should happen at merge time).