MyBB Source

I use chrome and I've encountered that plugin prob also.
I removed the editor (only a textfield and an input button is visible).

You can try TinyMCE to replace the default MyBB editor and see if this fix these issues. Instructions here:
http://community.mybboard.net/thread-44328.html

firefox ftw!

You can expand the box...

dont worry

Try using the Advanced Profile plugin.

It has profile comments, and it makes user's profiles look much better and more organized.

Link: http://mybbsource.com/thread-3213.html

I made my own editor theme and it works fine know

Compatibilty view sucks and this happens with me too.Its a CSS Bug for IE yet sometimes also occur with Firefox and Chrome(I've experienced).