This is less about how to do it and more about what you're getting into. Making responsive skins is very time consuming and difficult, and why I'm just always lazy and tell my mobile users to use the Proboards mobile version. >x> You'll probably want something like
this (I'm sure there's a Firefox version out there too) so you can get an idea of what it looks like at different resolutions. Just some things to consider:
On small screens, those sidebars are going to get really, really narrow if you make them fully responsive. Are you planning on putting content in there? Further, are they fixed? If they are, remember that people can't scroll them and will thus be unable to read any content if it falls off their screen. Paying attention to how tall the content is at the sidebar's thinnest is important. There is a min-width property just like there's a max-width one, so you can stop that to an extent, but that leads into the next issue.
If you put in some min and max widths, it might lead to a case where the boards overlap the sidebar. This means that some content is covered up. Are you okay with this? Do you have a way to deal with that so people can still get the information they need?
You'll want to make sure a reasonable amount of text is on each line. I think that's no less than 45 characters and no more than about 120ish, but I don't remember the exact numbers off the top of my head. So you'll want to check that on your narrow and wide options. Your forums are a little on the wide side on my screen (my screen is basically a small TV), but it's at levels I think would still be readable for me.
If you're making a responsive skin, you pretty much never want to set anything in pixels (IE, don't use width: 100px). You'll want to use
viewport,
em or percent so that all your padding, text, etc scales with the forum and doesn't look odd.
Responsive design is pretty hard. If you want an easier alternative so you can use some fixed width designs, there's always the option to put a toggle in for the sidebars. This way people with narrow screens can hide them when they're not looking at them. But these kinds of projects are good. If you can pull it off you'll have something pretty great.