It's b0rking when trying to report a network error to you. It's happening while expanding entities (
&netInterrupt.longDesc; denotes an "entity") within the XML document template that it uses to generate the page displaying the network error it encountered. This is a long standing Firefox bug, on the Mozilla tracker it is Bug 569229, and on the Tor Project tracker it is #8842.
From the Mozilla bugtracker:
This is triggered by nested entities. Expat keeps a linked list of open entities (openInternalEntities) which goes from the most inner open entity to the most outer open entity. In this case we're parsing an entity and it ends in another (inner) entity. While parsing that inner entity we block the parser, either because we're loading something like script, or because we interrupt. The inner entity hasn't been completely parsed, so it's not marked closed and the handler is set to internalEntityProcessor, the outer entity is considered to have been parsed completely, so it is marked closed and openInternalEntities is set to null. When we unblock the parser we end up in internalEntityProcessor with a null openInternalEntities, which is a bogus state and we report an error.
So, this is generated when Firefox is trying to render the XML for the page, the XML parser gets into a bad state and reports the error. There is a patch, but it seems it has not yet been introduced (the bug is still open on the Mozilla tracker). Note that this bug is from (at least) 2010, it is not a sign of compromise, just a sign of the (over-)complexity of XML.