Ten Mysteries of about:config

It's possible for a Web page to implement a denial-of-service attack on the browser user. All you need is a Web page that runs a heap of JavaScript in an infinite busy loop. Firefox can't accept user input when such intensive processing is going on. This integer preference causes script execution to halt if it goes on too long. The units are seconds, and the default is 5:

dom.max_script_run_time

You might have Firefox set up to do some tricky Web spidering. You might have it acting as a bot or running continuously as an unattended console. In any of those cases, set this preference to -1, and Web page scripts run forever unmolested.

The use of various asynchronous mechanisms, such as setTimeout(), support long scripting timelines in a normal Web page. There's no need for preference changes to support such things.

In the Firefox browser, a tab title, Location Bar URL or displayed bookmark can acquire a small image (an icon), which is displayed to its left. Usually it contains a brand mark for the site of the currently displayed Web page. You might not want this to happen. It makes your bookmarks file bigger, and (especially if you're on dial-up) it causes an extra HTTP request when the page first is visited. That request fetches the icon for display. These two Boolean preferences, both with a default of true, can be set to false to disable those fetches and the subsequent icon display:

browser.chrome.site_icons
browser.chrome.favicons

Set either one to false, and these icons are ignored. Bookmarks get the standard bookmarks icon, and elsewhere no icon at all appears.

You might ask, “Why are there two preferences?” Part of the reason is because these icons can be specified in two ways. You can put a 16x16 pixel Microsoft Windows ICO format icon at this URL: http://www.example.com/favicon.ico.

That icon will do for all pages on that site and is officially a Favorites Icon or favicon, to use Microsoft's term. Alternately, you can add an icon per page, using a <link> tag and any 16x16 ICO URL, like this:

<link rel="SHORTCUT ICON" href="/images/mybrand.ico" />

For some historical reason, that per-page use is called a Site Icon.

The other reason for two preferences has to do with parallel development streams in the Mozilla Project, the mess that is bookmark file formats and a shortage of time for trivial cleanup tasks. We're looking under the hood, remember.

If you have a drop of graphics programming in your blood, you might spare a kind thought for L. David Baron, Robert O'Callahan and company—the core developers of the Gecko rendering engine inside Firefox. Displaying a Web page is a fiendish compromise between standards, performance and subjective user perceptions. One of the most difficult constraints that Web pages impose is the need for incremental display. Show me the Web page as it arrives, not all at once at the end. This means constantly reflowing the displayed elements, which may be delivered out of order (a problem word processors don't have). Worse, such documents nearly always are network-delivered with unreliable timing.

To see the difficulty of this job, visit an image-intense Web site such as gamespot.com. Over broadband, the loading page jumps around in an agony of layout updates while chunky content is dumped into the browser in no particular order. On dial-up, the process is slower and more familiar, but the amount of layout labour is even larger, because there's more time to adjust each received fragment of page. Figure 2 shows the image-heavy GameSpot site, rendered while the images are still coming in.

Figure 2. Close-up of a half-received Web site, jumping around as Firefox updates the layout.

Given this kind of problem, you can imagine, therefore, that all kinds of hidden tuning preferences are available—if you know where to find them. This isn't a tuning workshop, so here are two of the more interesting ones.

It's rare to want to tune down Firefox. (You should buy your nice mother a better computer.) It's more likely that you've got lots of CPU and video grunt and want to use it. You probably click the mouse more than 2,000 times a day. Theoretically, you can shave a quarter of a second off your response time—that's an extra coffee break a day—with this integer preference:

nglayout.initialpaint.delay

Set this to 0 (zero) milliseconds. It's set to 250 by default. When a Web page starts to trickle in to the browser, Firefox normally waits a bit after it has organised the page fragments in memory. It makes sense to bunch up the first few bits of content before attempting to show them. If you've got a quick eye, though, you can make it show what it's got ready straight away.

Similarly, Firefox buffers up the incoming raw network content before it bothers to break those bytes down into something ready for display. That's another chunking process that saves the CPU but slows the output on a fast machine. Set this integer preference to, say, 5,000 (microseconds), and incoming network bytes are pushed to the display system much more quickly:

content.notify.interval

Doing so, however, makes Firefox work very hard scheduling updates in response to every drop of content. If you lower this value too much, that extra work merely results in the incoming data buffering further back in the dataflow—perhaps behind a socket in the kernel—while Firefox thrashes around trying to complete a whole display update for every trivial character of text that appears. Lower the preference a bit at a time, and watch the CPU with top(1), perhaps.