Unicode in irfanview

Here that range is mostly filled by Deja Vu Sans, STIXGeneral, VL Gothic, UnDotum, and... "MiscFixed", whatever that is (in the OCR section). Only holes at U+244A, U+24EB through U+24FF, U+269D, U+26BA through U+26BC, U+26C0 through U+26C3, and U+27CA remain.

If you look closely at your screen shot of Opera, you’ll see that there are problems there, too. The characters that are missing on Firefox are missing on Opera, too, they are just replaced by various empty rectangles instead of rectangles with hex numbers in them. The answer to the question about differences is that the browsers handle the missing glyph problem differently.

Firefox actually handles the situation better by displaying an indication (albeit a technical one) about the identity of the character that cannot be rendered. CSS Fonts Module Level 3 CR recommends that, in section 5.2 Matching font styles: “8. If a particular character cannot be displayed using any font, the user agent should indicate by some means that a character is not being displayed, displaying either a symbolic representation of the missing glyph (e.g. using a Last Resort Font) or using the missing character glyph from a default font.”

As regards to the question why some characters are not rendered on either browser, check out the answers to the questions you mentioned. The general answer is that you simply need suitable fonts. For example, the character U+A78D mentioned by @mgkrebbs in a comment is relatively new (Unicode 5.2), and its font support is therefore rather limited. It has not yet been added to Arial Unicode MS, and it will probably never be added to Code2000 (which appears to be abandonware now). In practice, you would probably want to download and install the DejaVu font family to see it.