If you're not aware, the HTTP protocol specification[1] doesn't have a technical way of knowing ahead of time if it's a 1x1 tracking pixel.
So the remaining realistic options are:
(1) block ALL <img> tags downloads which then blocks any 1x1 tracking pixels
(2) allow <img> tags but block some (and maybe most but not all) 1x1 pixels via a blacklist of url domains (e.g. doubleclick.net) ... and/or heuristics based on the "style" attribute
The (1) already happens in many email clients that render HTML.
The (2) is happening with the ongoing cat & mouse game with AdBlock, EasyList, etc
Or Firefox could crowdsource the building of a bloom filter of the URLs for images that are 1x1. Or you could learn 1x1 tracking locally since the set of pages you visit will probably be similar if you just want some more general protection.
The bigger problem is that anything like this and the providers go up in size 1px at a time until it’s harder to distinguish from real content (at first transparent, then positioned off-screen, then overlays hiding it, then visible in a part of the page that doesn’t get as many views, dual-purposed with images/ads already on-screen, etc).
A better way is if Firefox just bundled an ad blocker and pushed ad blocking technology forward (eg more hooks to do expensive processing natively to save on power like Safari does). The challenge though is that something like 100% of their funding comes from an ad company.
Bloom filter wouldn't work. The bloom filter will tell you that a particular URL is "possibly" in the list. What do you do then? Reject it because it might be a tracking image?
Google pays mozilla in order to avoid a monopoly lawsuit. Just because it is good for mozilla, it doesn’t put any sort of incentive for them to follow.
What we probably need is a client-side neural net within the browser to notice that certain high information-content IMG URLs (i.e. those with random-looking garbage in the URL) from certain domains tend to result in very low information-content imagery. And that's the signature of a likely tracker.
You can get most of that with uBlock Origin right now! "Block large media elements" and set the limit ridiculously low. You now have to click to view images. Control this via settings and filter lists.
uBlock does not know the size until the file is requested. At that point it is too late, you have accessed the tracking file. That feature in uBlock is meant for people trying to save bandwidth/CPU. uBlock is great though, I would never browse the web without it.
Edit: I forgot the grandparent mentioned them. Ublock Origin implements the parent's proposal,so I mentioned it.
Focussing on file size is a mistake because you can just make the tracking image display something decorative or whatever. I was referring to a solution for not loading third party resources without manual approval, which would need to be more general than just blocking single pixels to be robust (although hueristics might be good enough for some cases)
I always wondered why mail providers don't load all images automatically the moment the mail is received and present a cached image to the users. Wouldn't that make tracking useless?
This helps in that you don't connect directly to the image so it doesn't leak your IP and other info that would be available from the connection.
Marketing emails still send unique URLs for each recipient so they can associate your email address with opening any images and links. Google's proxy doesn't remedy this.
As far as I know, Gmail doesn't load the images until the first time the user opens the message, so it unfortunately doesn't make the tracking as useless as you could hope.
Technically it loads all images through a Google proxy[0] which in theory prevents third parties from using images for tracking. The third parties only get a 'hit' when Google pulls the image into their cache, which is not when you open the mail.
Google can still track you, but if you really care about that then you're probably not using Gmail anyway.
Concrete-ish but still simple simple version: If you visit example.com, close the tab, and then load a page with an ad on it, both parties would love to show you an ad for whatever example.com is selling. If example.com has a tracking pixel for the ad domain, then this is trivial to make happen.
OK, I understand privacy concerns with actually private information like name/address/email/etc. But a tracking pixel for ads? Why is this a concern? Since when?
Would it be so bad if people actually worked their brains a bit and got smarter with online advertising (and lots of other stuff)?
This looks suspiciously like the situation with obesity. Instead of eating less to lose weight, and eating fresh to stay healthy, people just blame "the corporations" for all their troubles.
Seems to me like it's counterproductive. Just put up safety nets for people because they can't control themselves. What do you end up with? A bunch of impulsive idiots and a few organizations with way too much control over them.
This is reality as I see it. You can be offended if you want, but I'd suggest you learn not to be offended by some words on a screen. That of course requires responsibility, which is free but I can sell it to you for $99/month if you want.
No one is suggesting advertisers tracking you is bad because people "can't control themselves." People don't like being tracked because, over time, private information can be inferred from your viewing history. Things like your age, gender, relationship status, income, ethnicity, education, hobbies, health concerns, diet, voting preferences, family names, etc. can all be known about you with high certainty simply by having trackers on the sites you visit. Ironically, this is exactly the kind of data you understand being concerned about before making several bad faith assumptions about why people don't want to be tracked.
The transparent pixel tracking trick is common enough now that it should be blocked by default in all 'Private' modes.