This is definitely not the most direct approach. I'd like to see some people test it out so that maybe some super motivated can find a more specific workaround to whatever comcast's done with their most recent firewall update ;). 

I personally don't think it will cause security issues for most people due to windows / mac's built in firewalls, so don't disable your firewall.

DO NOT SUE ME IF SOMETHING GOES WRONG, YOU MADE THE DECISION TO LISTEN TO AN INTERNET STRANGER NAMED AFTER A FLOWER REPRODUCTIVE PART.

Workaround #1

How to fix the "can't connect to others issue" by putting your router in the DMZ (Xfinity routers only)

1. Open your command prompt (on Windows, this can be done by pressing WINDOWS + R and typing cmd in the prompt box that appears.)
2. Type ipconfig and press enter, this will show you a bunch of data.
3. Look for your IPv4 address, it should be in the form of 10.0.0.X, or 192.168.X.X leave this window open, you will need this for a future step
4. Open your browser and go to 10.0.0.1, this page allows you to change your router settings
5. Log in, the default information for an Xfinity router is admin as the username and password for the password
6. On the left-hand side there should be an expandable menu called "Advanced," expand that
7. Click on DMZ
8. Click "enable"
9. In the "IPv4 host" section, put in the numbers that you saw in ipconfig
10. Save your router settings

That's it. This worked for mystery author. The DMZ ostensibly makes your computer more vulnerable to attack, so it's not advisable to leave your router in it permanently. Just click disable when you're done playing if you are worried.