You mean "Helvetica"; but we're talking about the Georgia, serif font font stack for headers.

 

On Ubuntu, the equivalent font substitutions are Nimbus Roman No9 L  for "Times", and Tinos, Liberation Serif, Thorndale, and Thorndale AMT for "Times New Roman"; of the latter set I only have Liberation Serif (note it's not the Linux Libertine that Kaldari recommends), so I get that one. So the equivalent of 

font-family: "Helvetica Neue", "Helvetica", "Nimbus Sans L", "Arial", "Liberation Sans", sans-serif;

is something like:

font-family: "Georgia", "Times", "Linux Libertine", serif;

As I said when we discussed sans-serif font stack ([Wikitech-l] Should MediaWiki CSS prefer non-free fonts?), if we prefer the open source font (for the criteria Denis Jacquerye mentions), then we should list it before the proprietary font. The problem remains: getting designers and l10n advisers to install and verifiably use these open source fonts on Mac and Windows in order to make the call.

Nobody responded to http://jsfiddle.net/UPBUH/7/ with useful info on what Android, Mac, or Windows does, but it's still there if someone wants to add the serif fonts.