This is usually a corrupt file in thunderbird. These steps may fix the issue: - close thunderbird - just for safety, backup the profile, nomally at c:\users\<yourid>\appdata\roaming\thunderbird - find where thunderbird is installed, If using windows, rightclick on the thunderbird icon on desktop to do this. Then, DELETE the folder. - now, download desired release from thunderbird.net. I recommend downloading the same release you were using, either the ESR or the monthly. - install the new version and the XPCOM error should not be there.