Can you please update to Firefox 20.0.1? Firefox 12 is horribly out of date and insecure. Update Firefox to the latest release

Note that if web servers send an encoding via the HTTP response headers or an encoding is specified via a meta tag in the file that this default won't be used.

It is usually best to leave the encoding at the default Western ISO-8859-1 to avoid problems with accented characters in the 0x80-0x9f range.

Why do you want UTF-8 as the default?

I have version 21.0 not 12. Don't know where you got that from.

I go to a few story/book sites that for some reason does not display special characters. Only if you use unicode-8 do they show up.

It is likely that the server sends this file with the wrong encoding and possibly relies on other methods to force UTF-8 that do not work in Firefox.

Which encoding is selected?
See also Tools > Page Info > General

Do you get a ? in black diamond?

Can you post a link to a publicly accessible page that doesn't require authentication (signing on)?

I can't set the encoding either, if I remove ISO-8859-1, it gets added again automatically. Though thanks to cor-el's advice regarding See also Tools > Page Info > General

when the html of a page includes <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> I see the text in UTF8. I can thus see my Greek characters.

ozman812, maybe that's what you needed help with? I'm experimenting myself a bit with html these past days.

Note that Firefox will only use the encoding specified in a meta tag in case the server doesn't send an encoding via the HTTP response headers.
If the server sends an encoding and that encoding is wrong then only changing that encoding yourself (or via an extension) will help in such a case.

cor-el this all goes to your credit. I provide a modified answer because I want to show an example of how my language is displayed as gibberish because of this problem with character encoding.

Here's an example of the Greek word "Ειρήνη" which displays as "Ειρήνη" if the <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> tag is missing from the webpage. If the aforementioned tag is there, the text displays as "Ειρήνη" correctly. 

My motivation in this modified answer of yours cor-el is that I myself had problems with character encoding when experimenting with html code on my own Firefox 23.0 and opening my .html files without the above method with correct meta tags. I needed a modified answer because "changing that encoding myself" did not work and here's why: In my menu system I chose View menu-> Character Encoding-> Customize List-> The two items in that list are UTF-8 and ISO-8859-1-> I remove ISO-8859-1 leaving only UTF-8 in the list and click on the "OK" button. Now I quit firefox, I use top -b | grep firefox to make sure firefox isn't running in any way in the background on my system, now I start firefox and in View menu-> Character Encoding-> Customize List-> The 2 items are there again, like I never changed it. It did not store my preference in Character Encoding, so that's why I think this modification of mine is good.

Here is my modification of your answer:

Note that Firefox will only use the encoding specified in a meta tag in case the server doesn't send an encoding via the HTTP response headers. If the server sends an encoding and that encoding is wrong then only changing* that encoding yourself (or via an extension) will help in such a case.

• *modifying the markup/html of the page by including a <meta> tag that specifies Character Encoding.

Note that this is really a problem with the website. The server should be sending the correct encoding via the HTTP response header. If neither the server sends an encoding and there is also no meta tag or a BOM at the start of the file then the result is unpredictable. Changing the default encoding is also not recommended just like switching on auto-detect: universal.