If you want the Simplenote web interface to make the first line of a note a little larger (like it does in the Mac app) I made some user styles that handle that as well as style the note list’s scrollbar in Firefox.

https://gist.github.com/blakewatson/d55e47e34cd777f43271f6bdb803d4bb

If you use these with a custom style extension, make sure you use Stylus and not the official Stylish extension which is filled with tracking BS.