Markdown has been gaining a LOT of traction, and is extremely popular among developers, with a growing user base of professional writers as well. Sites such as the StackExchange Network, GitHub, Reddit, and Weasyl use it, and most blogging software either supports it, or has plugins for it. I believe Slack also supports it, and more services are always being added to that list.

That said, a shortcut bar would be nice for people not used to Markdown, or an option to export plain text files with BBCode into Markdown (perhaps Pandoc?).

Here is a screenshot in Firefox 46.0a2, along with the zoom level to the right of the search box. Zooming out to 90% brings everything into view, and makes the UI look a LOT better.

I have this same issue. I'm using Firefox 46a2 (Aurora/alpha channel) on Windows 7. However, I have no lower scroll bar to allow for horizontal scrolling.