For multiquoting, I just copy paste. I use Chrome and the mutlquote button doesnt work for me : p
But you know how when you hit the "reply with quote" button, their post is quoted in the text box that you write your post in? I just copy that, delete the quote, hit the "reply with quote" button for another post I want to reply to, paste the last quoted post above it, and if I wanna quote another post I copy both of them and then do it all again : p
Though, I dont do this often, cuz I have a bad habit of replying to a post I want to reply to immediately before checking the posts that came after it, so I often just make like 2-3 posts at once, haha : p