Bugzilla

Backed out for causing bc failures in browser_ext_devtools_inspectedWindow_targetSwitch.js 
* [Backout link](https://hg.mozilla.org/integration/autoland/rev/81e6553dcbd639f34537e8fd42e849e8c2cc85fe)
* [Push with failures](https://treeherder.mozilla.org/jobs?repo=autoland&group_state=expanded&fromchange=e2f56c91b9177e55e61ae438f7cfbdafefa811de&searchStr=linux%2C18.04%2Cx64%2Cwebrender%2Copt%2Cmochitests%2Cwith%2Cnetworking%2Con%2Csocket%2Cprocess%2Ctest-linux1804-64-qr%2Fopt-mochitest-browser-chrome-spi-nw%2Cbc5&tochange=69c8851b3faa54c18c294d2f6158986471501ea2&selectedTaskRun=BhJUhZ2gQ4eMamMbAffNRQ.0) 
* [Failure Log](https://treeherder.mozilla.org/logviewer?job_id=452663553&repo=autoland) 
* Failure line: TEST-UNEXPECTED-FAIL | browser/components/extensions/test/browser/browser_ext_devtools_inspectedWindow_targetSwitch.js | Test timed out -

Backed out for causing bc failures in browser_ext_devtools_inspectedWindow_targetSwitch.js 
* [Backout link](https://hg.mozilla.org/integration/autoland/rev/81e6553dcbd639f34537e8fd42e849e8c2cc85fe)
* [Push with failures](https://treeherder.mozilla.org/jobs?repo=autoland&group_state=expanded&selectedTaskRun=BhJUhZ2gQ4eMamMbAffNRQ.0&resultStatus=testfailed%2Cbusted%2Cexception%2Cusercancel&fromchange=e2f56c91b9177e55e61ae438f7cfbdafefa811de&searchStr=mochitest-browser-chrome&tochange=69c8851b3faa54c18c294d2f6158986471501ea2) 
* [Failure Log](https://treeherder.mozilla.org/logviewer?job_id=452663553&repo=autoland) 
* Failure line: TEST-UNEXPECTED-FAIL | browser/components/extensions/test/browser/browser_ext_devtools_inspectedWindow_targetSwitch.js | Test timed out -
TEST-UNEXPECTED-FAIL | browser/components/extensions/test/browser/browser_ext_devtools_inspectedWindow_targetSwitch.js | Uncaught exception received from previously timed out test bound  - at resource://testing-common/BrowserTestUtils.sys.mjs:538 - Error: The window unloaded while we were waiting for the browser to load - this should never happen.