As well as pure performance, it's important to consider the browsers' resource requirements. Although the CPU load is pretty consistent among the browsers, this is not the case with memory consumption.

In the first test, a web site is loaded and memory consumption measured. A second test measures the resources needed to load 10 tabbed sites. We chose sites without advertising, since the constantly changing content of such sites does not allow for reproducible results: for example, one time you may get a simple image, while the next time the page is loaded a Flash animation may appear.

With only one web site to display, the browsers use between 18.5MB and 35.3MB of memory under Mac OS X 10.5.2. Safari is the most memory-frugal, while Firefox is the biggest memory-hog. However, the picture changes if ten sites are loaded at the same time. Now, Firefox 3 Beta 5 leads the field with 76.2MB, followed by Safari with 93.7MB. Safari uses slightly more memory (98.1MB) if the STAND plug-in is installed. Opera 9.27 uses 134.5MB, while Opera 9.5 Beta uses 139.3MB — slightly more than Firefox 2.0.0.13 with 138.8MB.

Memory usage patterns vary widely under Windows Vista too. With only one website, Microsoft's browsers perform well: Firefox uses around twice as much RAM as Internet Explorer, while Opera and Safari use around three times as much. With ten sites loaded, the results are almost reversed. Now IE brings up the rear with 143.7MB, while Firefox 3 Beta 5 only uses 50.8MB. As with Mac OS, Firefox 3 Beta 5 is the most memory efficient. The Opera versions use 66.8MB (9.27) and 77.8MB (9.5 Beta), while Safari and Firefox 2.0.13, with 113.3MB and 94MB respectively, need much more memory — but still less than IE.

Memory use in megabytes: shorter bars are better.

Memory use in megabytes: shorter bars are better.