"Presentation of empty mi element"

<mi>T</mi>
<mi></mi>
<mi>e</mi>

With the above content, firefox leaves space between elements, "T e",
however mathjax renders it as "Te", with no space between them.
Specification does not say anything on how to display the content when
empty mi is present.

Specification allows for empty mi element to be present,

"An mi element with no content is allowed; <mi></mi> might, for example, be
used by an "expression editor" to represent a location in a MathML
expression which requires a "term" (according to conventional syntax for
mathematics) but does not yet contain one."