After installing Thunderbird Portable Version 52.1.1 on the Version
45.8.0 for Windows can not run (run) Thunderbird. The app
Failed at start giving the following error:
Error processing XML: entity not defined
Position: chrome: //messenger/content/messenger.xul
Line number 298, column 15:
This is usually an add-on that is telling Thunderbird it is compatible but isn't. Try starting in Safe Mode to determine what it is. Run ThunderbirdPortable.exe from the command line with -safe-mode after it:
Or you can copy ThunderbirdPortable.ini from ThunderbirdPortable\Other\Source to the ThunderbirdPortable directory, edit it, and change the AdditionalParameters line to read:
Thunderbird should start but with all addons disabled to allow you to troubleshoot.
Note that this issue is not related to the portable version. This is Thunderbird itself giving you this error.
Sometimes, the impossible can become possible, if you're awesome!
Thanks very much.
I deactivated the Google Calendar add-on and it worked.