First please make sure that Firefox for Android is up to date 40.0.3. Second, afaik the encryption of both are supported. About:config entry:

256: security.ssl3.dhe_rsa_aes_256_sha and security.ssl3.ecdhe_ecdsa_aes_256_sha etc (search for 255) 128: with ssl3 seems to rsa, rc4 and ecdhe

Need more help? Try [howsmyssl.com] on your mobile device.

256-bit encryption has been in use by Mozilla related stuff for several years if site supports it.

Is there a reason for your question? say a website telling you need a browser that supported 128-bit ?