Merge pull request #8787 from lightsofapollo/bug-839273-add-gaurds-for-removed-headers

Bug 854292  - Add more safegaurds for headers being deleted while they ha...(cherry picked from commit c3a1298ff0d6ec23d0f1aee4f195be7ffd6ca2fa)
Merge pull request #8983 from jrburke/worker-thread

Bug 814257 email: Move back-end into worker thread, r=asuth, r=jrburke, r=lightsofapollo, r=squib(cherry picked from commit 0649e5e9d2f71c96517bca1b582c706e665bca6a)