The only thing that $$...$$ has over [[math:...]] is in the situation of a very short formula like a single letter, and there it's better to use <var> instead of a gif *anyway*.
I disagree strongly. That would mean that if I have a formula with one letter variables and two letter variables the first would be written as <var> and the second as TeX within the same text. Always using LaTeX will look better and is more consistent.
Formulas should be seen as "inclusions", so I prefer the [[math: syntax overall. The software should look at the formula inside the brackets, and choose how to output it. In particular, if it detects Mozilla it should output MATHML instead of calling TeX. Maybe if it's just a variable name, it could just output a <var>.