How can I disable highlighting in html or JS?

You can use the CSS pseudo class selector ::selection and ::-moz-selection for Firefox.

For example:

::-moz-selection {
    background-color: transparent;
    color: #000;
}

::selection {
    background-color: transparent;
    color: #000;
}

.myclass::-moz-selection,
.myclass::selection { ... }

The CSS3 solution:

user-select: none;
-moz-user-select: none;

There is also a webkit prefix for the user-select, but it makes some form fields impossible to focus (could be a temporary bug), so you could go with the following pseudo-class for webkit instead:

element::selection