1. Visit https://www.facebook.com/events/birthdays/.
2. IMPORTANT: Scroll all the way down to the bottom to load the birthdays for the year.
3. While still on the Facebook birthdays page, open up a Javascript console by using a keyboard shortcut. In Chrome, use Option + ⌘ + J (on macOS) or Shift + CTRL + J (on Windows/Linux). A warning might show about this method being used for malicious purposes. Feel free to audit this code if you don’t trust it.

4. Paste in this snippet:

   // From https://raw.githubusercontent.com/nwcell/ics.js/master/demo/ics.deps.min.js, but with 'rrule' capitalized:
   var saveAs=saveAs||function(e){"use strict";if(typeof e==="undefined"||typeof navigator!=="undefined"&&/MSIE [1-9]\./.test(navigator.userAgent)){return}var t=e.document,n=function(){return e.URL||e.webkitURL||e},r=t.createElementNS("http://www.w3.org/1999/xhtml","a"),o="download"in r,a=function(e){var t=new MouseEvent("click");e.dispatchEvent(t)},i=/constructor/i.test(e.HTMLElement)||e.safari,f=/CriOS\/[\d]+/.test(navigator.userAgent),u=function(t){(e.setImmediate||e.setTimeout)(function(){throw t},0)},s="application/octet-stream",d=1e3*40,c=function(e){var t=function(){if(typeof e==="string"){n().revokeObjectURL(e)}else{e.remove()}};setTimeout(t,d)},l=function(e,t,n){t=[].concat(t);var r=t.length;while(r--){var o=e["on"+t[r]];if(typeof o==="function"){try{o.call(e,n||e)}catch(a){u(a)}}}},p=function(e){if(/^\s*(?:text\/\S*|application\/xml|\S*\/\S*\+xml)\s*;.*charset\s*=\s*utf-8/i.test(e.type)){return new Blob([String.fromCharCode(65279),e],{type:e.type})}return e},v=function(t,u,d){if(!d){t=p(t)}var v=this,w=t.type,m=w===s,y,h=function(){l(v,"writestart progress write writeend".split(" "))},S=function(){if((f||m&&i)&&e.FileReader){var r=new FileReader;r.onloadend=function(){var t=f?r.result:r.result.replace(/^data:[^;]*;/,"data:attachment/file;");var n=e.open(t,"_blank");if(!n)e.location.href=t;t=undefined;v.readyState=v.DONE;h()};r.readAsDataURL(t);v.readyState=v.INIT;return}if(!y){y=n().createObjectURL(t)}if(m){e.location.href=y}else{var o=e.open(y,"_blank");if(!o){e.location.href=y}}v.readyState=v.DONE;h();c(y)};v.readyState=v.INIT;if(o){y=n().createObjectURL(t);setTimeout(function(){r.href=y;r.download=u;a(r);h();c(y);v.readyState=v.DONE});return}S()},w=v.prototype,m=function(e,t,n){return new v(e,t||e.name||"download",n)};if(typeof navigator!=="undefined"&&navigator.msSaveOrOpenBlob){return function(e,t,n){t=t||e.name||"download";if(!n){e=p(e)}return navigator.msSaveOrOpenBlob(e,t)}}w.abort=function(){};w.readyState=w.INIT=0;w.WRITING=1;w.DONE=2;w.error=w.onwritestart=w.onprogress=w.onwrite=w.onabort=w.onerror=w.onwriteend=null;return m}(typeof self!=="undefined"&&self||typeof window!=="undefined"&&window||this.content);if(typeof module!=="undefined"&&module.exports){module.exports.saveAs=saveAs}else if(typeof define!=="undefined"&&define!==null&&define.amd!==null){define("FileSaver.js",function(){return saveAs})}
   var ics=function(e,t){"use strict";{if(!(navigator.userAgent.indexOf("MSIE")>-1&&-1==navigator.userAgent.indexOf("MSIE 10"))){void 0===e&&(e="default"),void 0===t&&(t="Calendar");var r=-1!==navigator.appVersion.indexOf("Win")?"\r\n":"\n",n=[],i=["BEGIN:VCALENDAR","PRODID:"+t,"VERSION:2.0"].join(r),o=r+"END:VCALENDAR",a=["SU","MO","TU","WE","TH","FR","SA"];return{events:function(){return n},calendar:function(){return i+r+n.join(r)+o},addEvent:function(t,i,o,l,u,s){if(void 0===t||void 0===i||void 0===o||void 0===l||void 0===u)return!1;if(s&&!s.rrule){if("YEARLY"!==s.freq&&"MONTHLY"!==s.freq&&"WEEKLY"!==s.freq&&"DAILY"!==s.freq)throw"Recurrence rrule frequency must be provided and be one of the following: 'YEARLY', 'MONTHLY', 'WEEKLY', or 'DAILY'";if(s.until&&isNaN(Date.parse(s.until)))throw"Recurrence rrule 'until' must be a valid date string";if(s.interval&&isNaN(parseInt(s.interval)))throw"Recurrence rrule 'interval' must be an integer";if(s.count&&isNaN(parseInt(s.count)))throw"Recurrence rrule 'count' must be an integer";if(void 0!==s.byday){if("[object Array]"!==Object.prototype.toString.call(s.byday))throw"Recurrence rrule 'byday' must be an array";if(s.byday.length>7)throw"Recurrence rrule 'byday' array must not be longer than the 7 days in a week";s.byday=s.byday.filter(function(e,t){return s.byday.indexOf(e)==t});for(var c in s.byday)if(a.indexOf(s.byday[c])<0)throw"Recurrence rrule 'byday' values must include only the following: 'SU', 'MO', 'TU', 'WE', 'TH', 'FR', 'SA'"}}var g=new Date(l),d=new Date(u),f=new Date,S=("0000"+g.getFullYear().toString()).slice(-4),E=("00"+(g.getMonth()+1).toString()).slice(-2),v=("00"+g.getDate().toString()).slice(-2),y=("00"+g.getHours().toString()).slice(-2),A=("00"+g.getMinutes().toString()).slice(-2),T=("00"+g.getSeconds().toString()).slice(-2),b=("0000"+d.getFullYear().toString()).slice(-4),D=("00"+(d.getMonth()+1).toString()).slice(-2),N=("00"+d.getDate().toString()).slice(-2),h=("00"+d.getHours().toString()).slice(-2),I=("00"+d.getMinutes().toString()).slice(-2),R=("00"+d.getMinutes().toString()).slice(-2),M=("0000"+f.getFullYear().toString()).slice(-4),w=("00"+(f.getMonth()+1).toString()).slice(-2),L=("00"+f.getDate().toString()).slice(-2),O=("00"+f.getHours().toString()).slice(-2),p=("00"+f.getMinutes().toString()).slice(-2),Y=("00"+f.getMinutes().toString()).slice(-2),U="",V="";y+A+T+h+I+R!=0&&(U="T"+y+A+T,V="T"+h+I+R);var B,C=S+E+v+U,j=b+D+N+V,m=M+w+L+("T"+O+p+Y);if(s)if(s.rrule)B=s.rrule;else{if(B="RRULE:FREQ="+s.freq,s.until){var x=new Date(Date.parse(s.until)).toISOString();B+=";UNTIL="+x.substring(0,x.length-13).replace(/[-]/g,"")+"000000Z"}s.interval&&(B+=";INTERVAL="+s.interval),s.count&&(B+=";COUNT="+s.count),s.byday&&s.byday.length>0&&(B+=";BYDAY="+s.byday.join(","))}(new Date).toISOString();var H=["BEGIN:VEVENT","UID:"+n.length+"@"+e,"CLASS:PUBLIC","DESCRIPTION:"+i,"DTSTAMP;VALUE=DATE-TIME:"+m,"DTSTART;VALUE=DATE-TIME:"+C,"DTEND;VALUE=DATE-TIME:"+j,"LOCATION:"+o,"SUMMARY;LANGUAGE=en-us:"+t,"TRANSP:TRANSPARENT","END:VEVENT"];return B&&H.splice(4,0,B),H=H.join(r),n.push(H),H},download:function(e,t){if(n.length<1)return!1;t=void 0!==t?t:".ics",e=void 0!==e?e:"calendar";var a,l=i+r+n.join(r)+o;if(-1===navigator.userAgent.indexOf("MSIE 10"))a=new Blob([l]);else{var u=new BlobBuilder;u.append(l),a=u.getBlob("text/x-vCalendar;charset="+document.characterSet)}return saveAs(a,e+t),l},build:function(){return!(n.length<1)&&i+r+n.join(r)+o}}}console.log("Unsupported Browser")}};
   
   var cal = ics();
   re = /^(?<name>.*) \((?<month>.*)\/(?<day>.*)\)/gm;
   var d = new Date();
   var year = d.getFullYear();
   document.querySelectorAll("li a").forEach(function(person) {
       tooltip = person.dataset.tooltipContent;
       if (typeof tooltip !== 'undefined') {
           matched = re.exec(tooltip)
           if (matched !== null) {
               bd = matched.groups;
               console.log(tooltip);
               console.log(bd);
               day = `${bd.month}/${bd.day}/${year}`
               cal.addEvent(`${bd.name}'s birthday`, '', '', day, day, {freq: 'YEARLY', count: 200});
           }
       }
   })
   cal.download('Facebook birthdays');
   

5. Press Enter.
6. Add the ICS file to your calendar system of choice.

Note that this has a slight issue where nearby birthdays may not appear. Particulary the ones under “Today’s birthdays”, “Recent birthdays”, and “Upcoming birthdays”. You can add those manually to your calendar.

Of course, some may not want all birthdays on their calendar. It’s always possible to hide the calendar or edit the birthdays that appear after adding the calendar to your favorite service.