I have set up Mantis to use utf8 in my language (Czech). The RSS feeds come in the following format:

BOM (newline)
<?xml ...... etc.

This is, of course, wrong, there should be no newline after the UTF-8 BOM at the beginning of the file (I think that XML spec says that the <?xml ...> header must be on the first line of the file). So Thunderbird refuses to read this feed. To reproduce the issue, try setting the charset in your language file to utf8. I even tried replacing the RSS feed generator with the most recent version, but no luck... so I think that it's Mantis fault.