Hacking Firefox: The secrets of about:config Stop memory hogging

Stop memory hogging

The default way the Windows version of Firefox consumes memory can be alarming if you don't know what's really going on. People routinely report a memory "footprint" of 75MB to 100MB or more with only a few windows or tabs open, and they assume a memory leak is to blame. While earlier versions of Firefox did have memory leak bugs, they're not the reason for this kind of memory consumption in Firefox 2.x.

Here's what's happening: Firefox caches recently used objects -- Web pages, images -- in memory so that they can be re-rendered on-screen quickly, which drives up memory usage. The following tweaks can make Firefox stake out memory less aggressively. (Note, however, that lightening the memory load might make your pages load a bit more slowly than you're used to.)

Reduce graphics caching
When the Boolean preference browser.cache.memory.enable is enabled (the default), Firefox keeps copies of all graphical elements from the current browsing session in memory for faster rendering. You can set this to false to free up more memory, but pages in your history will reload less quickly when you revisit them.

Another option: Set the value to true and create a new integer preference called browser.cache.memory.capacity. Then specify, in kilobytes, how much memory to set aside for graphics caching. That way you get some of the speed benefits that graphics caching provides without taking a huge memory hit. If you use -1 as the memory value, Firefox will size the memory cache based on how much physical RAM is present.

Reduce Web page caching
Firefox caches several recently visited Web pages in memory so they don't have to be regenerated when you press Back or Forward. The integer setting browser.sessionhistory.max_total_viewers determines how many individual Web pages to store in the back/forward cache; each page takes about 4MB (or 4,000KB) of RAM.

By default, however, this value is set to -1, which determines how many pages to cache from the amount of available physical memory; the maximum number of pages stored when you use -1 is 8. Set this value to 0 to disable page caching entirely. That will save some memory, but will also cause Back and Forward navigation to slow down a bit.

Note that this caching is not the same as browser.cache.memory.enable: That setting is for rendering elements on pages like graphics and buttons, and the contents of https-encoded pages, while this setting is for caching the text content of Web pages that have already been rendered or "tokenized."

Swap out to disk memory when minimized (Windows only)
A little-known feature in Firefox allows the Windows memory manager to swap out some of Firefox's physical memory space to disk when Firefox is minimized but not closed. This allows other programs to use the physical memory that Firefox was previously monopolizing.

Firefox's minimized memory usage with (top) and without (bottom) config.trim_on_minimize. (Click for larger view.)

By default, this feature is turned off, for two reasons: 1) PC memory is generally more plentiful than it used to be, so it makes sense to use it if it's available, and 2) swapping Firefox's memory out to disk will slow the program down when it's restored.

That said, if you run Firefox side by side with other memory-hungry applications, it might help keep them from competing with each other. To enable this feature, create a new Boolean preference called config.trim_on_minimize and set its value to true.

Got your own about:config tweaks to share? Add them to the Comments area at the bottom of the page. If you've got the itch to learn more about about:config settings, MozillaZine's about:config entries wiki is a great source of information.