Instiki uses MathJax to render mathematics on browsers that don’t support MathML (either natively, or through a plugin). That’s slow (and, with mixed MathML/SVG content, somewhat buggy) so it’s preferable to have a browser that supports MathML and SVG natively.
You will likely need to install some fonts in order to render MathML correctly. N.B.: those instructions have changed for Gecko 31 and later.
If you happen to have installed the TeX Live distribution, there was an option to install the Latin Modern fonts for system-wide use. If you have the Latin Modern Math font installed on your system, that will suffice. Otherwise:
The MathPlayer plugin provides MathML support for IE < 11. Apparently, it no longer works in IE 11 or later.
Whatever you installed, head over to Jason Blevins’s test site, to give your new MathML-enabled installation a workout.