Developers! Remember that you MUST not print any data in browser before call print_header() because it prevent to create right page headers.
For example, we lost "charset" and "Content-Language".
So, please check following:
1. All files which UTF-8 encoded should be WITHOUT signature.
2. Nothing will be before <?php at begin of file (it's simple )
3. No empty line at end of file! (this presents in some files. For example: block_wiki_index.php, block_wiki_ranking.php)
4. Nothing should print before print_header()! Need to examine program logic (see wikitimeline.php for example)
This is FireFox view: