Make Firefox2 faster, easy customizations

This post started life in 2008 as a list of Firefox 2 about:config tweaks. Many major versions later, most of those tweaks are gone: HTTP pipelining was removed outright because HTTP/2 made it pointless, the tab-width and close-button preferences disappeared with the UI redesigns, and a lot of what we used to hand-tune is now simply the default. What follows is the modern version: how about:config works today, which of the old ideas still apply, and where Firefox actually needs help when it feels slow.

Opening about:config

Type about:config in the address bar and confirm the warning page — Firefox shows it because a wrong value here can genuinely break the browser. Current versions apply changes immediately; a restart is rarely needed anymore. Every value you’ve changed shows a reset button, so undoing an experiment is one click. The configuration editor works the same way on desktop platforms; the old advice about copying prefs.js by hand is replaced by about:profiles, which lets you open and back up your profile safely.

Preferences still worth changing

• network.prefetch-next → false. Stops Firefox from speculatively downloading pages it thinks you will visit next. Still documented by Mozilla as the way to disable link prefetching; it saves bandwidth and avoids requests for pages you never actually open.
• browser.startup.page → 3. Restores your previous session — tabs and all — on every launch. The same behavior is available as “Open previous windows and tabs” in Settings → General → Startup, which is the friendlier way to set it.
• browser.backspace_action → 0. Makes Backspace navigate back a page. Firefox’s default today is to scroll the page instead, but plenty of people prefer the old behavior.

Old tweaks that are gone — and what replaced them

• HTTP pipelining (network.http.pipelining): removed from the browser entirely. HTTP/2 multiplexes many requests over one connection and does the job properly — there is nothing left to set.
• Tab width and close buttons (browser.tabs.tabMinWidth, browser.tabs.closeButtons): removed years ago. Tab scrolling and per-tab close buttons are fixed behavior now.
• More simultaneous downloads: the original reason I opened about:config in 2008. Today the download manager already handles multiple files in parallel, and per-server HTTP connections are capped at six by default (network.http.max-persistent-connections-per-server). Raising it just moves the bottleneck to the server — leave it alone.

When Firefox actually feels slow

• Check the built-in Task Manager first (Shift+Esc). It shows per-tab and per-extension CPU and memory usage, and a runaway tab or extension explains most “Firefox is slow” reports.
• Settings → General → Performance: leave “Use recommended performance settings” enabled unless you have a specific reason to touch hardware acceleration or the content process count.
• If things are genuinely broken, Refresh Firefox from about:support resets the browser to defaults while keeping bookmarks, passwords, and history — the modern replacement for deleting prefs.js and hoping.

The meta-lesson from comparing the 2008 list with today: browser defaults got good. The tweaks that survived are privacy and bandwidth choices, not speed hacks — modern Firefox tunes itself, and when it misbehaves the answer is usually an extension or a runaway tab, not a preference.