Firefox 41 uses less memory to run AdBlock Plus. Nicholas Nethercote

Last year I wrote about AdBlock Plus’s effects on Firefox’s memory usage . It was this part that I found the most significant.

First, you will notice a continuous overhead of 60-70 Mb just by enabling ABP. […] Most of this appears to be due to JavaScript memory consumption, though some may also be due to layout memory.

A second overhead of approximately 4 MiB per iframe can be attributed to ABP injecting stylesheets in each iframe. Multiple iframes can be added to pages quickly. Firefox can use about 194 MiB in physical memory, for example, when I load TechCrunch. ABP increases that amount to 417 Mb.

this page, with over 400 iframes, is another extreme example. Firefox is able to use about 370MB without ABP. With ABP that number leaps to around 1960 MiB.

(The description of the overhead was not exact; it is per document. The overhead includes both high-level documents in tabs as well documents in embedded iframes.

Mozilla developer Cameron McCormack released patches last week to address bug 77999 that was filed more 14 years back. These patches make it possible to share CSS-related CSS data. They add data structures which allow you to share the result of cascading CSS style sheets.

As an example, the above-mentioned extreme example (also known. According to the Vim Color Scheme Test, memory usage decreased by 3.62MiB per document. You can see that there are now 429 pages on this page. This represents an approximate decrease of 1,550 miB. The memory usage on that page is down to about 350 MiB. All these numbers are based on 64-bit versions.

Other sites were also measured and I was able to confirm the constant saving of approximately 3.6 MiB each document when AdBlock Plus has been enabled. Since the page count varies greatly from one page to another, this effect is dependent on your workload. TechCrunch’s main page has been changed in a way that it doesn’t trigger excessive memory usage. In one example I attempted opening four articles in each cnn.com , and bbc.co.uk pages for one measurement. It took me 15 tabs to open the frontpage. Cameron’s patches removed Firefox used approximately 90 MiB more physical memory. This represents a decrease of almost 10%.

AdBlock Plus does not need to be turned on, so this option has moderate benefits. Vim Color Scheme Test for instance, reduced memory usage of documents by 0.09 MiB. That’s about 40 MiB less.

This Nightly Firefox release and AdBlock Plus version will be required to verify the changes. Because of a new Regressionrelated JavaScript, AdBlock Plus older versions won’t run with Nightly. On Firefox’s about.memory page, you will see the diminution in the “style sets” measurements. You’ll also see a new entry under “layout/rule-processor-cache”, which is the measurement of the newly shared data; it’s usually just a few MiB.

Firefox 41 will include this improvement, and it is expected to be released on September 22 2015. Firefox 41 Beta, which is available for download on August 11, is planned to be released on the other release channels. Firefox 41 Developer edition will release within the next few days.