Revert "Merge pull request #19753 from mnjul/bug_962902_duplicate_blob_url_in_system_app"

This reverts commit 053de51fcb7d1cd0553d3dd092e1974fda62a1f3, reversing
changes made to e98fe1e94d3d80ad36903500d8ca3333904b162c.
Revert "Merge pull request #19765 from wilsonpage/1008187"

This reverts commit 392123a9c53fd9a6ccbb1c81908c3176c0647222, reversing
changes made to 54d6512f516564c23e4363ca328b2a453605617b.
Bug 1018393 - Disable intermittent failing test, failing test, test_a11y_keyboard_key_names.py test_a11y_keyboard_key_names.TestA11yKeyboardKeyNames.test_a11y_keyboard_key_names
Bug 1013929 - Use the languagechange event instead of a mozSettings observer. r=stas