https://www.eff.org/HTTPS-EVERYWHERE is an absolute must-have browser extension for this.

I have this extension and Comcast. I didn't even notice they were injecting things

I don't have that extension, I do have Comcast, and I don't see injected Comcast content. Either uBO or another blocker (belt and suspenders) is blocking that, or they don't do it in my region, or my block, or whatever.

They only inject when you're on "xfinitywifi" networks and if you've pirated something and they want to inform you that you've been caught, iirc.

Not true. They've been injecting ads to my android phone which is served by a wireless device I own. And I haven't pirated anything. I had to switch to Firefox with ublock to fix it.

Unfortunately when hitting a data limit that causes these Comcast pop ups it breaks HTTPS connections. The only way to regain internet access is to visit a HTTP site such as example.com to get the pop up.

I've had this experience too. I complained to the FCC about it, and for my efforts got a likely-cookie-cutter mail response from comcast explaining that they provide these "rare, important messages" "for my own good".

edit: to clarify, I was complaining about the fact it broke all my connections, not that it was going on in the first place (which i'm also livid about)