Why is ​Firefox Quantum so fast? Mozilla reveals a tweak that turbo-charged its browser

Mozilla's latest version of its Firefox web browser gets a performance boost from a privacy feature.
Written by Liam Tung, Contributing Writer

Video: Firefox Quantum will give you a faster browser but there's a price to pay

A Mozilla engineer has revealed one of the hidden techniques that Firefox 57 -- known as Quantum -- is using to improve page load times.

Mozilla has already flagged a number of architecture changes to Firefox Quantum that delivered speed and performance improvements, but Firefox 57 has another secret trick to achieve faster page loads: 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.

The feature isn't intended as a privacy enhancement, although it does rely on data from Firefox's privacy feature Tracking Protection.

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 doesn't work well on some sites though, in some cases causing a page to be blank for a few seconds instead of gradually building the page. Bambas points to Google's Page-Hiding Snippet as an example where this can occur.

Also: Firefox Quantum: A cheat sheet for professionals

However, as he notes in a bug report related to Page-Hiding Snippet, "the intention of tailing is to help performance by delaying something that is assumed to not have any visible effect".

Firefox is also moving ahead with plans to mark all HTTP pages as insecure. Currently it shows warnings in input fields on HTTP pages that handle login or financial data. As BleepingComputer reports, the Firefox 59 Nightly build can be configured to show that any site that is not HTTPS as insecure.

The first step is to provide an option to make Firefox display a non-secure indicator for all HTTP sites. At some point in future, as the web gets closer to ubiquitous HTTPS, Firefox could begin labelling HTTP sites as non-secure by default.

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