When I converted a userstyle that contains regexp rule using backslashes into "To Mozilla format" userstyle, its first backslash became double.
For example, http://userstyles.org/styles/65020/regexp-test-style.
a userstyle that contains regexp rule using backslashes
"To Mozilla format" userstyle
On Stylish 0.10, Chrome 18.0.1025.168, Windows 7 Home Premium SP1 64bit.
Actually, all backslashes are supposed to be doubled when doing this because your regexp value needs to be escaped using CSS rules. So it's correct that the first backslash is doubled, but incorrect that the rest aren't.