Why does Firebug say toFixed() is not a function?

That is because Low is a string.

.toFixed() only works with a number.

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Try doing:

Low = parseFloat(Low).toFixed(..);

toFixed isn't a method of non-numeric variable types. In other words, Low and High can't be fixed because when you get the value of something in Javascript, it automatically is set to a string type. Using parseFloat() (or parseInt() with a radix, if it's an integer) will allow you to convert different variable types to numbers which will enable the toFixed() function to work.

var Low  = parseFloat($SliderValFrom.val()),
    High = parseFloat($SliderValTo.val());