@implementation MyAppDelegate

+ (void)initialize
{
    NSString* useragent = @"Mozilla/5.0 (Windows NT 6.3; rv:36.0) Gecko/20100101 Firefox/36.0";
    NSLog(@"Setting User-Agent to: %@", useragent);
    NSDictionary* dictionary = [NSDictionary dictionaryWithObjectsAndKeys:useragent, @"UserAgent", nil];
    [[NSUserDefaults standardUserDefaults] registerDefaults:dictionary];

}
(...)

(...)
 [request setValue:@"Mozilla/5.0 (Windows NT 6.3; rv:36.0) Gecko/20100101 Firefox/36.0" forHTTPHeaderField:@"User-Agent"];
 [webView loadRequest: request];
 NSLog(@"Getting UA: %@", [webView stringByEvaluatingJavaScriptFromString:@"navigator.userAgent"]);
 (...)

Getting UA: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:31.0) Gecko/20130401 Firefox/31.0/Worklight/7.0.0.0