Do the following steps:

Step 1: In a new tab, type or paste "about:config" in the address bar and press Enter. Click the button promising to be careful.

Step 2: In the list, find or search field above the list, type or paste "dhe" and pause while the list is filtered

Step 3: Double-click the security.ssl3.dhe_rsa_aes_128_sha preference to switch it from true to false (disable Firefox from using this cipher)

Step 4: Double-click the security.ssl3.dhe_rsa_aes_256_sha preference to switch it from true to false (disable Firefox from using this cipher). Then reload the page using Ctrl+Shift+R