To remove window decorations for all or
particular applications, use the <decor> option in the <applications> section of rc.xml (user: ~/.config/openbox/ or system: /etc/xdg/openbox/).
Example for Firefox, including variants like Firefox-Beta and Firefox-Nightly:
<application class="Firefox*">
<decor>no</decor>
</application>
One could also disable decorations for all applications (using class "*"), then enable them (using yes) for individual ones. To apply the changes, restart your desktop session, and thus Openbox. Reference: Openbox FAQ