Oh, yes, that'd be because pixiv will redirect you back to mode=medium if the referrer isn't medium already, and the script just grabs the first image on the page to upload.
Posted under General
Oh, I should've been clearer: I know perfectly well what the cause is (what I don't actually know is why Chrom(e/ium) doesn't get a 403). I just didn't have the time to work on it yet, and think of / test the best strategy, since just loading the fullsize in an iframe fully would make it significantly slower in operation than only fetching the HTML over XHR. So I need to play with various ways that could convince FF to prime its cache (and the headers sent) without actually fetching the image, and that requires time I don't have right now.
Yes, see forum #78245.
New pastebin that includes the fix: http://pastebin.com/cSRx2jfQ
I edited the Danbooru Upload Bookmarklet to work with Danbooru2. You can get V1.5 here: http://pastebin.com/3kTdr59h
The functions are the same as the previous V1.41.