With FF 11, it was some obscure change with CORS xmlHTTPRequests that worked on FF 10, but not anymore on FF11. I am not talking about a plugin or anything like that, just a web site that worked under FF 10 but not anymore on FF 11.
Just like what the author of the article is complaining about, TinyMCE (as the version used on some sites he needs) stopped working with FF11.
You can always claim that this is not the fault of FF, but at the point where FF is not able to display the sites you rely on consistently after every other upgrade, you know you need to switch to another browser that does not require web sites updates every time the browser is updated.
This (along with plugins) is the best way to loose users. We used to tell our customers to use FF as the preferred browser, we now advocate Chrome. This is just less trouble for us and our customers. Too bad.