When you paste some text to Grammarly editor in Firefox, initially the browser displays a security dialog box with two buttons: 'Allow' and 'Deny'. If you click (or have clicked) 'Allow', everything works fine. If you click (or have clicked) 'Deny' and 'remember this decision', you'll have to adjust Firefox settings to be able to paste the text. Here's how to do this:

1. Type "about:config" in your address bar. 
2. Click "I promise". 
3. Enter "signed.applets.codebase_principal_support" in the filter field.  
4. Change the "false" meaning to "true".