since I installed FF58 I get the following error.

XML Parsing Error: undefined entity
Location: chrome://browser/content/browser.xul
Line Number 245, Column 5:    <broadcaster id="viewTabsSidebar"
autoCheck="false" sidebartitle="&syncedTabs.sidebar.label;"


After a fresh installation the browser initially starts normally for each user
on the system. But from the second start on FF stops working with above error
message.

I have deleted the complete .mozilla folder, but to no avail.

Safe-mode works. The cleaning function in safe-mode does not solve the issue
though.

Rolling back to FF57 everything runs smoothly again.