Please note that the website is responsible for telling Firefox what character encoding to use.

The label name has changed to "Text Encoding".

See:

• View > "Text Encoding"

I would like to set text encoding to UTF-8. Where to do it for Firefox?

You can no longer do this. There is only a general Western or Unicode choice.

You can look here to set a fallback character encoding: Tools > Options > Content > Fonts & Colors > Advanced > Character Encoding for Legacy Content

• [/questions/987964] Legacy encoding: UTF-8 no longer available as of Firefox 28