If you can't add attachments from your computer to more than one email in your OWA email using Firefox, below is an option to try.
Turn off the option to type new messages in a new window.
1. From the settings menu (the gear icon), select Options.
2. Select Mail on the left.
3. Select Layout, then Reading pane.
4. Change the reading pane option to Type in the reading pane, then click the Save button at the top.