The telemetry added in bug 952543 has recorded a few exceptions (in order of number of occurrences in the last three-ish weeks of Nightly builds): 305 {"message": "[Exception... \"Component returned failure code: 0x8000ffff (NS_ERROR_UNEXPECTED) [nsIPrefBranch.setIntPref]\" nsresult: \"0x8000ffff (NS_ERROR_UNEXPECTED)\" location: \"JS frame :: resource://gre/modules/AddonManager.jsm :: AMI_startup :: line 503\" data: no]", "module": "AMI", "context": "startup failed"} 75 {"context": "startup failed", "message": "ReferenceError: id is not defined", "module": "XPI", "line": 2407, "file": "resource://gre/modules/XPIProvider.jsm"} 14 {"message": "[Exception... \"Component returned failure code: 0x80004005 (NS_ERROR_FAILURE) [nsIFile.moveTo]\" nsresult: \"0x80004005 (NS_ERROR_FAILURE)\" location: \"JS frame :: resource://gre/modules/XPIProvider.jsm -> resource://gre/modules/XPIProviderUtils.js :: XPIDB_writeAddonsList :: line 1481\" data: no]", "module": "XPI", "context": "startup failed"} 7 {"message": "[Exception... \"Component returned failure code: 0x80520015 (NS_ERROR_FILE_ACCESS_DENIED) [nsIFile.moveTo]\" nsresult: \"0x80520015 (NS_ERROR_FILE_ACCESS_DENIED)\" location: \"JS frame :: resource://gre/modules/XPIProvider.jsm -> resource://gre/modules/XPIProviderUtils.js :: XPIDB_writeAddonsList :: line 1481\" data: no]", "module": "XPI", "context": "startup failed"} 5 {"context": "startup failed", "message": "ReferenceError: id is not defined", "module": "XPI", "line": 2415, "file": "resource://gre/modules/XPIProvider.jsm"} 4 {"context": "startup failed", "message": "ReferenceError: id is not defined", "module": "XPI", "line": 2423, "file": "resource://gre/modules/XPIProvider.jsm"} 2 {"message": "[Exception... \"Component returned failure code: 0x80520001 (NS_ERROR_FILE_UNRECOGNIZED_PATH) [nsIFile.persistentDescriptor]\" nsresult: \"0x80520001 (NS_ERROR_FILE_UNRECOGNIZED_PATH)\" location: \"JS frame :: resource://gre/modules/XPIProvider.jsm :: XPI_startup :: line 2072\" data: no]", "module": "XPI-BOOTSTRAP", "context": "startup failed"} 1 {"message": "[Exception... \"Component returned failure code: 0x80520012 (NS_ERROR_FILE_NOT_FOUND) [nsIFile.isDirectory]\" nsresult: \"0x80520012 (NS_ERROR_FILE_NOT_FOUND)\" location: \"JS frame :: resource://gre/modules/XPIProvider.jsm :: XPI_processPendingFileChanges/< :: line 2456\" data: no]", "module": "XPI", "context": "startup failed"} 1 {"message": "[Exception... \"Component returned failure code: 0x80004005 (NS_ERROR_FAILURE) [nsIFileOutputStream.write]\" nsresult: \"0x80004005 (NS_ERROR_FAILURE)\" location: \"JS frame :: resource://gre/modules/XPIProvider.jsm -> resource://gre/modules/XPIProviderUtils.js :: XPIDB_writeAddonsList :: line 1479\" data: no]", "module": "XPI", "context": "startup failed"}

The most common error: [Exception... \"Component returned failure code: 0x8000ffff (NS_ERROR_UNEXPECTED) [nsIPrefBranch.setIntPref]\" nsresult: \"0x8000ffff (NS_ERROR_UNEXPECTED)\" location: \"JS frame :: resource://gre/modules/AddonManager.jsm :: AMI_startup :: line 503\" data: no] Based on reading the code, the most likely cause for this is that the stored preferences file defines the 'extensions.blocklist.pingcountversion' pref to be something other than Int. I can't see anything else that would lead to this error.

This should either fix or catch&log all the exceptions except the first (setIntPref). We should do something better with the writeAddonsList() failures, but I'd prefer to leave that for when we convert it to async I/O. In other news, I hate it when my warning-logging code throws. What's Latin for "Who exception-catches the exception-catchers?"