Please post the instructions so we can check it out.

Configuring Filelink

To use Filelink, you must first create a new account (or configure an existing account) with an online storage service provider.

   At the top of the Thunderbird window, click the Tools menu (ALT+T) and select Options or click the Application menu button New Fx Menu and choose Options .
   Click on the Attachments panel, and choose the Outgoing tab.
   Click the Add button.
   Select the desired service provider from the drop-down list.
       Choose Box, click Set up Account, or click the Get a Box account… link to create a new account. 
   Click Set up Account to continue.

Is the problem with Thunderbird? I've called the big guys to help you. Good luck.

its for emailing large files through thunderbird but firefox is thunderbird.

Filelink sounds like an exploit, and may not be valid anymore. A simple fix is to use 7zip or it's equal, and compress the file(s) and then send the now smaller file. Or you could use other tricks. They take a long time to set up though. It involves multiple programs and I would not suggest it to someone who doesn't want to spend a week figuring out how to do it. That said if you want to create an infinite email/cloud/encrypt/free service. You can find them but, even I do not bother. Cheers

I went to the thunderbird menu/ preferences/attachments and was able to do set it up.