Last year I wrote about AdBlock Plus’s effect on Firefox’s memory usage. The most important part was the following.

First, there’s a constant overhead just from enabling ABP of something like 60–70 MiB. […] This appears to be mostly due to additional JavaScript memory usage, though there’s also some due to extra layout memory. Second, there’s an overhead of about 4 MiB per iframe, which is mostly due to ABP injecting a giant stylesheet into every iframe. Many pages have multiple iframes, so this can add up quickly. For example, if I load TechCrunch and roll over the social buttons on every story […], without ABP, Firefox uses about 194 MiB of physical memory. With ABP, that number more than doubles, to 417 MiB. An even more extreme example is this page, which contains over 400 iframes. Without ABP, Firefox uses about 370 MiB. With ABP, that number jumps to 1960 MiB.

(This description was imprecise; the overhead is actually per document, which includes both top-level documents in a tab and documents in iframes.)

Last week Mozilla developer Cameron McCormack landed patches to fix bug 77999, which was filed more than 14 years ago. These patches enable sharing of CSS-related data — more specifically, they add data structures that share the results of cascading user agent style sheets — and in doing so they entirely fix the second issue, which is the more important of the two.

For example, on the above-mentioned “extreme example” (a.k.a. the Vim Color Scheme Test) memory usage dropped by 3.62 MiB per document. There are 429 documents on that page, which is a total reduction of about 1,550 MiB, reducing memory usage for that page down to about 450 MiB, which is not that much more than when AdBlock Plus is absent. (All these measurements are on a 64-bit build.)

I also did measurements on various other sites and confirmed the consistent saving of ~3.6 MiB per document when AdBlock Plus is enabled. The number of documents varies widely from page to page, so the exact effect depends greatly on workload. (I wanted to test TechCrunch again, but its front page has been significantly changed so it no longer triggers such high memory usage.) For example, for one of my measurements I tried opening the front page and four articles from each of nytimes.com, cnn.com and bbc.co.uk, for a total of 15 tabs. With Cameron’s patches applied Firefox with AdBlock Plus used about 90 MiB less physical memory, which is a reduction of over 10%.

Even when AdBlock Plus is not enabled this change has a moderate benefit. For example, in the Vim Color Scheme Test the memory usage for each document dropped by 0.09 MiB, reducing memory usage by about 40 MiB.

If you want to test this change out yourself, you’ll need a Nightly build of Firefox and a development build of AdBlock Plus. (Older versions of AdBlock Plus don’t work with Nightly due to a recent regression related to JavaScript parsing). In Firefox’s about:memory page you’ll see the reduction 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.

This improvement is on track to make it into Firefox 41, which is scheduled for release on September 22, 2015. For users on other release channels, Firefox 41 Beta is scheduled for release on August 11, and Firefox 41 Developer Edition is scheduled to be released in the next day or two.