Copy the base64 encoded certificate text that starts with "-----BEGIN CERTIFICATE-----" and ends with "-----END CERTIFICATE-----" to the clipboard after having selected the full text with the mouse.
Open a plain text editor like Notepad and paste the certificate text of the intermediate certificate that you have placed on the clipboard in the editing area.
Use "Save File as" and set the File type to "All files" and save the certificate text to a .cer file.
Select "All files" when saving the file to avoid getting a hidden .txt file extension (.cer.txt) appended.
Import the saved certificate in the Firefox Certificate Manager.