This combines kennebec's (K) answer with Hermann Ingjaldsson's (H) answer:
navigator.browserSpecs = (function(){
var ua = navigator.userAgent, tem,
M = ua.match(/(opera|chrome|safari|firefox|msie|trident(?=\/))\/?\s*(\d+)/i) || [];
if(/trident/i.test(M[1])){
tem = /\brv[ :]+(\d+)/g.exec(ua) || [];
return {name:'IE',version:(tem[1] || '')};
}
if(M[1]=== 'Chrome'){
tem = ua.match(/\b(OPR|Edge)\/(\d+)/);
if(tem != null) return {name:tem[1].replace('OPR', 'Opera'),version:tem[2]};
}
M = M[2]? [M[1], M[2]]: [navigator.appName, navigator.appVersion, '-?'];
if((tem = ua.match(/version\/(\d+)/i))!= null)
M.splice(1, 1, tem[1]);
return {name:M[0], version:M[1]};
})();
console.log(navigator.browserSpecs); //Object { name: "Firefox", version: "42" }
if (navigator.browserSpecs.name == 'Firefox') {
// Do something for Firefox.
if (navigator.browserSpecs.version > 42) {
// Do something for Firefox versions greater than 42.
}
}
else {
// Do something for all other browsers.
}