Okay, I think I’ve got a good ‘work-around/bug-fix’ [firefox seems to be interpreting the atom+xml mime type definition a bit differently than I — and they are probably right, considering the definition is written gobbledeegook]
Anyway, the patch — edit the file ‘includes/default-filters.php’
Near the bottom, you will see a section like this:
// RSS filters
add_filter('the_title_rss', 'strip_tags');
add_filter('the_title_rss', 'ent2ncr', 8);
add_filter('the_content_rss', 'ent2ncr', 8);
add_filter('the_excerpt_rss', 'convert_chars');
add_filter('the_excerpt_rss', 'ent2ncr', 8);
add_filter('comment_author_rss', 'ent2ncr', 8);
add_filter('comment_text_rss', 'htmlspecialchars');
add_filter('comment_text_rss', 'ent2ncr', 8);
add_filter('bloginfo_rss', 'ent2ncr', 8);
add_filter('the_author', 'ent2ncr', 8);
Add:
add_filter('the_category_rss', 'ent2ncr', 8);
(and make sure to reload your ‘live’ firefox bookmark — open the bookmark menu, and right-click on the entry for your feed)