Most important settings in optimizing Mozilla Firefox, K-Meleon and other Gecko-based browsers

Usually these two about:config instructions rarely appear together in any entry about optimizing a Gecko-enabled browser, such as Mozilla Firefox, K-Meleon, SeaMonkey and others.

In about:config —
• network.prefetch-next — toggle to false (can be done with a doubleclick)
• browser.sessionhistory.max_total_viewers — change to 0 on computers with low resources or 2 on friskier machines.
In very modern and very fast machines, the default settings can be kept intact.

Update: 28 Coolest Firefox about:config Tricks — Contains instructions both for Windows and Linux users and those who prefer some of the older Firefox UI features.

E., 18.07.2011. Update: Something I very recently found by chance in a version of Firefox on Knoppix 4.0.2, yet which may well exist in other systems and versions:

browser.related.enabled can be set to false, because the preference does very little specifically in anything that is not SeaMonkey (unless someone is using any What's Related-like extension that uses the preference).

Most of the preferences well apply to older versions of Gecko-based browsers, especially those that use Gecko 1.7, 1.8 and 1.8.1 (upwards to Firefox 2, SeaMonkey 1.1 and K-Meleon 1.5.4).

Firefox 4.0/5.0+ (where 5.0 is actually an incremental development because of rapid development practices now at mozilla.org) are less and less likely to require these modifications, because they're typically installed on modern and very modern computers.

L., 22.10.2011. Update:

browser.sessionstore.max_concurrent_tabs — Set to 0

If you're using multiple tabs, then upon session restore all the tabs are restored, but pages are loaded only on the tabs that you open. If you're using more than one window, the last tabs open before closing will still load.

Applies to Mozilla Firefox/Iceweasel/IceCat 4–8 only
08.01.2012. update: The feature was deprecated in version 9.0 of Firefox and corresponding forks. Users who still have version 8.0.1 at most and are happy with how it performs, should turn off the auto-update feature in Preferences.

Other options can be found from about:config entries at MozillaZine Knowledge Base Wiki.

P., 04.03.2012. Update:
This one should be useful for users of LiveCD-s:

plugins.hide_infobar_for_missing_plugin

This should work in Mozilla Firefox/Debian Iceweasel/GNU IceCat 3.0 and SeaMonkey 2.0 or newer (given that the function was first checked in on 02.10.2007).

P., 05.08.2012. Update: browser.sessionstore.interval — Set integer to more than 10000 (milliseconds; default value means 10 seconds) — I sometimes set it to 34000 or even 60000.

This can be useful if you don't want Firefox to interfere with plugin work. (I won't guarantee that it would work as expected, but in theory it sould improve performance. Apply if you think it may help.)