Symbola certainly does support most of the characters used in that article (not all though). Specifically, it doesn’t have glyphs for the CJK characters or the regional indicator symbols used on the page, so you’ll need another font for those. (OS X v10.7+ includes the 'Apple Color Emoji' font which has glyphs for the regional indicator symbols, while on Windows 7, 'Segoe UI Symbol' includes glyphs for them (you need to have installed the update¹ for the font first though)).
Anyways, firstly, there’s no need to uncheck 'Allow pages to choose their own fonts, instead of my selections above'. You can go ahead and let the page specify whatever fonts it wants. If you don’t have the font(s) specified in the webpage’s stylesheet, or the webpage contains a character that the current font doesn’t have a glyph for, your OS/browser will substitute it for another font on your system that does have a glyph for that character (if it exists), so you can recheck that option. In fact, you don’t have to explicitly even pick Symbola to be used as a font for any type of text in Firefox at all, since your OS should use font substitution automatically if any of those fonts chosen there don’t have a glyph for a character on whatever webpage you’re on. In fact, to begin with, it’s impossible for any one font to contain all of Unicode right now, since even OpenType fonts can only contain a maximum of 65,536 glyphs, while Unicode has more than 120,000 assigned codepoints, so font substitution is absolutely necessary (so you can change the fonts in Firefox back to the defaults if you like).
Secondly, after you install the font, you may have to restart the computer or close Firefox before it actually picks up on the new font.
Thirdly, the version of Symbola that was linked to in the article is an old one. I’d recommend this² one instead (covers more codepoints).
Anyways, firstly, there’s no need to uncheck 'Allow pages to choose their own fonts, instead of my selections above'. You can go ahead and let the page specify whatever fonts it wants. If you don’t have the font(s) specified in the webpage’s stylesheet, or the webpage contains a character that the current font doesn’t have a glyph for, your OS/browser will substitute it for another font on your system that does have a glyph for that character (if it exists), so you can recheck that option. In fact, you don’t have to explicitly even pick Symbola to be used as a font for any type of text in Firefox at all, since your OS should use font substitution automatically if any of those fonts chosen there don’t have a glyph for a character on whatever webpage you’re on. In fact, to begin with, it’s impossible for any one font to contain all of Unicode right now, since even OpenType fonts can only contain a maximum of 65,536 glyphs, while Unicode has more than 120,000 assigned codepoints, so font substitution is absolutely necessary (so you can change the fonts in Firefox back to the defaults if you like).
Secondly, after you install the font, you may have to restart the computer or close Firefox before it actually picks up on the new font.
Thirdly, the version of Symbola that was linked to in the article is an old one. I’d recommend this² one instead (covers more codepoints).
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¹ — https://support.microsoft.com/en-us/kb/2729094
² — https://web.archive.org/web/20150625020428/http://users.teil...