Comment 2 for bug 1433259

It's possible for any of the other fields to have values specific to each data type too. The logical conclusion would be that we'd have not just "MMS proxy" and "MMS proxy port", but also "MMS user name", "MMS password", and eventually perhaps "MMS authentication" and "MMS protocol". That would be redundant. Instead, we just have "Custom MMS APN" -- and soon just "MMS APN" -- as the title for the dialog.

iOS uses the label "MMS Proxy" because its MMS APN settings are on the same screen as its Internet APN settings. And Android uses the labels "Proxy", "MMS proxy", and "MMS port" on the same screen because it lets you configure "an APN" which you then use for both Internet and MMS, and they might have different proxy values. Neither of those apply here.