When using Earthlink email I cannot reply to a message. I could for the first 6 months I used it but about 6 weeks ago the problem arose. How do I fix it?

I use email through a site called TheGrid.net which is a part of Earthlink.net. It worked fine with Firefox till about six weeks ago. Now when I am looking at an email message when I hit Reply or when I try to write a message, it won't let me. It brings up two blank boxes below the message heading. It won't let me enter data in the first box and when I type in the second box, which it will let me do, it will not accept it when I hit Send. I suspect you did an automatic update to Firefox when the problem arose. Can you help? Thank you.