Since these lines are almost exactly the same, not sure if you want to venture down the path of RegExp (ie: `/(Tablet|Mobile)/.test(ua) && ua.indexOf('Firefox') > -1;`, or since `indexOf('Firefox')` is calculated twice, move it out.

Something like this hot mess of untested mess:
```javascript
var isFirefox = ua.indexOf('Firefox') > -1;
var isMobileOrTablet = /(Mobile|Tablet)/.test(ua);

return ! (isFirefox && isMobileOrTablet);
```

Or heck, cheat and if it _is_ Firefox, just return `false` early and not even bother with regexes.

Or whatever. Maybe IE8 has poor/no RegExp support (shocker) and it isnt worth it. I was only making wild speculations from this lacklustre chart: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/test