If your Mozilla Firefox web browser can no longer access certain HTTPS websites, try this fix

Mozilla Firefox version 39 onwards will by default block the HTTPS connections to web servers using weak ephemeral Diffie-Hellman (DH) keys.

Bear in mind that by default, Firefox is set to automatically update itself from time to time, so your version will eventually be updated to version 39 and above.
This is a security measure against the Logjam attack. You can click here to read more about this Logjam attack.
If you visit to an affected website using HTTPS connection with Firefox, you will see the following error message:

Affected websites including the webmail of older version of Zimbra Collaboration Suite. Click here for more detail.
You can fix this by installing the Disable DHE 0.1.2 Add-on by Mozilla to Firefox.

To install, go to Menu > Add-ons in Firefox, and search for "Disable DHE". Once you found the add-on, click the Install button to install it.

Restart Firefox and check if this fix works for you.