*** UPDATE:: After several hours, this problem fixed itself, as follows: I decided to run a Norton scan. However, I could not open the Norton program, even though its icon showed it as running and happy. I fixed this by running Norton's Remove and reinstall process, which among other things performed a minor update to the Norton software. A couple of restarts later, Norton was working happily and I performed a Full Scan, which revealed no problems. Then I opened Thunderbird, which automatically installed 38.0.1 and downloaded all my emails. So perhaps there had been some conflict when the Norton upgrade and the Thunderbird upgrade both ran automatically at startup. But NOTE: I still DO NOT HAVE CHROME.MANIFEST, so there seems to be room for improvement to the Mozilla upgrade or at least to the error messages. Beginning this morning I am unable to send or receive emails. My Thunderbird version is 31.7.0 and I am on the Release Update Channel. Checking for updates tells me that I am up to date. My environment is Windows 8.1 with Norton Internet Security. My wife's PC, which (to the extent of my limited skills) is configured identically to mine, is on Thunderbird 38.0.1, and can send and receive emails. The full error console messages are (1) "Could not read chrome manifest 'file:///C:/Program%20Files%20(x86)/Mozilla%20Thunderbird/chrome.manifest" (2) "Could not read chrome manifest 'file:///C:/Program%20Files%20(x86)/Mozilla%20Thunderbird/extensions/%78972ce4c6-7e08-4474-a285-3208198ce6fd%7D/chrome.manifest (3)mutating the [[Prototype]] of an object will cause your code to run very slowly; instead create the object with the correct initial [[Prototype]] value using Object.create resource://gre/components/steel/Application.js