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After updating: XML Parsing Error: undefined entity Location: chrome://contactssidebar/content/contactsPanel.xul Line Number 210, Column 9:

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Several of my end users started getting the error message after updating recently. XML Parsing Error: undefined entity Location: chrome://contactssidebar/content/contactsPanel.xul Line Number 210, Column 9:

Several of my end users started getting the error message after updating recently. XML Parsing Error: undefined entity Location: chrome://contactssidebar/content/contactsPanel.xul Line Number 210, Column 9:

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Are they using the Contact Sidebar add-on? Looks like this 10-year-old project is DOA in TB 52:

https://addons.mozilla.org/en-us/thunderbird/addon/contacts-sidebar/reviews/882514/

https://forums.opensuse.org/showthread.php/525042-Thunderbird-problem-after-update

What features are desired? They may be available in other add-ons.