View source tabs don't include the status bar line / col indicator that the windows had. Additionally, for text files like resource://gre/modules/addons/XPIProviderUtils.js, we aren't showing line numbers in the gutter. With both of these together, there's no way to get line number info for text files. We should bring back some kind of line info for them. I think numbers in the gutter (like many editors) would be best.

I agree we should have line numbers in the gutter at a minimum. Here's a screenshot of Chrome's View Source for the following plain text: https://bug1253798.bmoattachments.org/attachment.cgi?id=8727032