Well, it doesn't really matter what you enter as a mobile string since the plugin won't actually behave like the selected browser but just 'tell' the server that it is another browser.
In other words; just enter ANY mobile browser string to have your WordPress blog present the mobile content. FireFox will then display this content being FireFox, not how the actual 'faked' browser would handle it.
Take a look at http://www.useragentstring.com/pages/useragentstring.php for a pretty complete list of useragents. Personally I use this string I got from my mobile phone...
Mozilla/4.0 (compatible; MSIE 6.0; Windows CE; IEMobile 7.11)
...you could use this one too. Again; it's just a way of 'triggering' the mobile content from your blog. It will ALWAYS look the same, no matter WHAT mobile user-agent you use, as long as it's recognized as being a mobile user-agent. :-)