the screen keeps jumping back to the left after closing the side menu

Yep, same here, getting this on page load. It leaves a huge white space to the right side of the page.
Just use this CSS snippet in e.g. Stylish to fix that in the mean time:

@namespace url(http://www.w3.org/1999/xhtml);

@-moz-document domain("pixiv.net") {
    #wrapper {
        margin: 10px auto 0 !important;
    }
}

I see, thanks.
Let's hope it'll properly work in the actual FF36 release!

Hi,
I'm using Firefox Developer Edition (36.0a2 (2014-12-28)) and GreaseMonkey 2.3.
Upon arriving on Pixiv's main page, or on search pages (haven't tested anything else) I'm getting these errors several times a second in the browser console:
TypeError: hotbox.parentElement is undefined %255BTS%255D_Pixiv++.user.js:1910:12 TypeError: hotbox is undefined %255BTS%255D_Pixiv++.user.js:1910:12
Sometimes it's the hotbox, sometimes it's hotbox.parentElement that's undefined. Simply switching tabs or hovering elements can cause the change.

Here's a gif showing a bit of it:

Thanks!