Somebody asked a question on Discord about whether the markdown editor and preview windows could swap position. I have no idea if this will break anything (as it isn't something I will ever use and have no real impetus to test it) but I managed to do it by adding the following to userchrome.css.
.rli-editor > div > div > div > div > div > div {
flex-direction: row-reverse !important;
}
Et voilà!
