For some websites I noticed the Firefox's 'View/character encoding' menu is disabled, why?
Is it possible to enable it?
It's due to the fix for Bug 234628, Disable View>Character Coding menu when it won't have effect / is unnecessary (e.g. XML).
Specifically, if you look at comment #63:
The BOM is used to identify the order of bytes in a Unicode-encoded document.
The reasons given can be summarised as:
The example page you provided starts with the three UTF-8 BOM bytes. You can see this if you save the page and open the HTML file in a hex editor. The BOM positively identifies it as a UTF-8 document, and opening it in any other encoding would not yield a correct HTML page.
If you really want to, you can save the file, strip off the offending BOM and open the file. Or you can set up a proxy (Fiddler2 is good for this) that will intercept and modify the files before they get to your browser. However, these aren't really good solutions, and are likely to just create further problems. The best thing to do, if you have encoding issues, is to contact the website maintainer. We should all be moving to Unicode where possible anyway, not using the older and more limited encoding standards.