Are you sure? I'm running Firefox 38.0.5 and Chromium 40.0.2214.91 through normal channels (ie, not nightly builds...) and they are both NOT vulnerable.
https://bugzilla.mozilla.org/show_bug.cgi?id=1166031 and https://bugzilla.mozilla.org/show_bug.cgi?id=1138554 say explicitly wontfix for 38.0.5
Maybe you manually fixed it some time ago and don't remember?
Check both https://weakdh.org/ and https://www.ssllabs.com:10445/
Secure Connection Failed
An error occurred during a connection to 10.xx.yy.xx. SSL received a weak ephemeral Diffie-Hellman key in Server Key Exchange handshake message. (Error code: ssl_error_weak_server_ephemeral_dh_key)
- The page you are trying to view cannot be shown because the authenticity of the received data could not be verified.
- Please contact the website owners to inform them of this problem.
Both on Mac and OpenSuSE.
- Chrome 43.0.2357.124
- Firefox 38.0.5
- Safari 8.0.6 (10600.6.3)
Not vulnerable
- Opera 30.0.1835.59
Reference: https://www.ssllabs.com/ssltest/viewMyClient.html