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Firefox 57's Speed Secret? Delaying Requests from Tracking Domains (zdnet.com)
An anonymous reader quotes ZDNet:
A Mozilla engineer has revealed one of the hidden techniques that Firefox 57 -- known as Quantum -- is using to improve page load times... It delays scripts from tracking domains, such as www.google-analytics.com. The technique was developed by Mozilla engineer Honza Bambas, who calls it "tailing". It works by delaying scripts from tracking domains when a page is actively loading and rendering...
Tailing only briefly prevents the tracking scripts loading, rather than disabling them entirely. Page load performance is improved by saving on network bandwidth and computing resources while loading a page, in a way that prioritizes site requests over tracking requests. "Requests are kept on hold only while there are site sub-resources still loading and only up to about 6 seconds. The delay is engaged only for scripts added dynamically or as async. Tracking images are always delayed. This is legal according all HTML specifications and it's assumed that well built sites will not be affected regarding functionality," explains Bambas.
Tailing only briefly prevents the tracking scripts loading, rather than disabling them entirely. Page load performance is improved by saving on network bandwidth and computing resources while loading a page, in a way that prioritizes site requests over tracking requests. "Requests are kept on hold only while there are site sub-resources still loading and only up to about 6 seconds. The delay is engaged only for scripts added dynamically or as async. Tracking images are always delayed. This is legal according all HTML specifications and it's assumed that well built sites will not be affected regarding functionality," explains Bambas.
FireFox inherited a small security update from the Tor project called "First Party Isolation". It's in newer versions of FF, but isn't turned on by default as it can break some authentication systems.
What it does, is only allow cookies to be sent and received by the site in the page's URL. So, for instance, while visiting YouTube.com, images and the like from google.com can load, but have no cookies attached, and do not receive those cookies.
To enable it, go to about:config and find "privacy.firstparty.isolate". Set it to true and restart the browser, and enjoy surfing the web knowing that you're not being tracked from site to site.