That came to my mind too, but I don't think it's doable thanks to XSS restrictions.
To prevent nefarious scripts, XSS (cross-site scripting) isn't allowed by browsers. Thus, when you're on a posterous.com page, javascript can only make requests to posterous.com. When you're on a twitpic.com page, javascript can only make requests to twitpic.com. The way sites get around this is by making the request to the external site from their servers, but then the calls would still come from Posterous servers, so it would get blocked.
Why the hate for firefox? I support firefox more, it started all the improvements in browsers, is done by a small company, and doesn't have so many fingers into my privacy pie.
I wouldn't say Firefox "started" all the improvements ... Opera has been at the forefront for a long time, natural progression has taken place over the years, and now Firefox is playing catch-up with things like tab-isolation and HTML5
To prevent nefarious scripts, XSS (cross-site scripting) isn't allowed by browsers. Thus, when you're on a posterous.com page, javascript can only make requests to posterous.com. When you're on a twitpic.com page, javascript can only make requests to twitpic.com. The way sites get around this is by making the request to the external site from their servers, but then the calls would still come from Posterous servers, so it would get blocked.