The trim() function uses this regex:

/^(\\s|\\u00A0)+|(\\s|\\u00A0)+$/g

This is intended to handle both normal whitespace characters, as well as the Unicode character for non-breaking spaces. (\\u00A0 or charcode 160).

This regex triggers the "unresponsive script" error in Firefox and Chrome when run on a string that has several successive non-breaking-space characters in the middle (not leading or trailing). It appears to be the part for trailing whitespace that is triggering this bug in the browsers.

This breaks:

// Create a string with a bunch of non-breaking spaces in the middle
var arr = []; for (var i = 0; i < 30; i++) { arr.push(160); }
var str = 's' + String.fromCharCode.apply(null, arr) + 's';
// Try to right trim with JQuery's regex
str.replace(/(\\s|\\u00A0)+$/g, '');

Doing it in two passes works:

// Create a string with a bunch of non-breaking spaces in the middle
var arr = []; for (var i = 0; i < 30; i++) { arr.push(160); }
var str = 's' + String.fromCharCode.apply(null, arr) + 's';
// Try to right trim with JQuery's regex
str.replace(/(\\s)+$/g, '').replace(/(\\u00A0)+$/g, '');

One way to fix would be for the trim() function to do the combined replace for leading whitespace, then to do the trailing replace in two passes.