For some reason the forum thinks it should automatically create a URL if you end a sentence with ftp.

Also does the same if you end a sentence with www.

I would say it's not aggressive enough, unless it also does it for sentences ending in gopher.

You can use a unicode zero-width space to avoid the urlification. That is, enter ftp​.

Here's how it looks: ftp​.

Making this automatic with a greasemonkey script is left as an exercise for the reader.

On another, related note, why is it the forum software accepts & but not &? (the latter is escaped into &)

phpBB is free-ninety-nine. That's why.