Type about:preferences#general<enter> in the address bar. Across from fonts and colors, press the Advanced button. On the bottom, turn on Allow Web Sites To Choose Their Own.

Fonts Information - Detected via Flash http://browserspy.dk/fonts-flash.php?detail=1

Also see: https://support.mozilla.org/en-US/search/advanced?q=fonts&language=en-US&product=firefox&sortby_documents=relevance&a=1&w=1

Do you have a font installed that covers this CJK Unicode range ?

You may see little boxes with the hex code of the character if you do not have a font installed that covers this Unicode range.

See also:

• https://en.wikibooks.org/wiki/Unicode/Character_reference/C000-CFFF

Do you have a font installed that covers this CJK Unicode range ? You may see little boxes with the hex code of the character if you do not have a font installed that covers this Unicode range. See also:

• https://en.wikibooks.org/wiki/Unicode/Character_reference/C000-CFFF

well, it happened all of sudden and it shows properly 70% of the time and it doesn't 30% of the time. So I don't think it's the problem with the CJK unicode font. But thanks for the suggestion.

Type about:preferences#general<enter> in the address bar. Across from fonts and colors, press the Advanced button. On the bottom, turn on Allow Web Sites To Choose Their Own. Fonts Information - Detected via Flash http://browserspy.dk/fonts-flash.php?detail=1 Also see: https://support.mozilla.org/en-US/search/advanced?q=fonts&language=en-US&product=firefox&sortby_documents=relevance&a=1&w=1

It's been already turned on but it's the same. The thing is it happens only sometimes and it started to happen all of sudden.

Do you have a font installed that covers this CJK Unicode range ? You may see little boxes with the hex code of the character if you do not have a font installed that covers this Unicode range. See also:

• https://en.wikibooks.org/wiki/Unicode/Character_reference/C000-CFFF

How should I install the right font then? I don't think it's the font problem though since these boxes show up only sometimes and I just don't know when it happens and when doesn't.

What font is specified and what font is Firefox trying to use (if any) ?

You can right-click and select "Inspect Element" to open the builtin Inspector with this element selected.

• https://developer.mozilla.org/en-US/Tools/Page_Inspector/How_to/Examine_and_edit_HTML

You can check in the Rules tab in the right panel in the Inspector what font-family is used for selected text. You can check in the Font tab in the right panel in the Inspector what font is actually used because Firefox might be using a different font than specified by the website.

• https://developer.mozilla.org/en-US/Tools/Page_Inspector/How_to/Edit_fonts