This is a theme conflict. I can't help here, just click on left space before [A] icon and it'll be opened. You can't change wpForo editor.

How the f... you know all those little secrets....

What I'll do without your brain - thank you! 

I'll try an official WP theme to check for a perfect compatibility, I'm open for suggestions...  

Follow-up,

I've changed the theme to WP official Twenty Ten and I have the same issue.

Is it possible a RTL problem? Seems that the down-arrow is there on the left but invisible.

Thank You. 

Maybe this is a RTL issue. Please navigate to Forums > Settings > Styles admin page and put this CSS code in "ustom CSS Code" textarea. Then delete cache and do Ctrl+F5 on forum page:

#wpforo #wpforo-wrap .mce-btn-small i {line-height: 20px;vertical-align: top;}
#wpforo #wpforo-wrap .mce-btn .mce-caret {margin-right: 0;margin-top: 8px;}
#wpforo #wpforo-wrap .mce-btn-small .mce-caret {margin-right: 0; margin-top: 8px;}
#wpforo #wpforo-wrap .mce-caret {border-right: 4px solid transparent; border-left: 4px solid transparent; border-top: 4px solid #333 !important; content: ""; display: inline-block; height: 0; vertical-align: top;width: 0;}
#wpforo #wpforo-wrap .mce-caret:hover { background-color:inherit;}
#wpforo #wpforo-wrap .mce-disabled .mce-caret {border-top-color: #aaa;}