Optimizing Firefox for a weak PC

This blogpost was first written as a reply on reddit.
This post applies to Firefox Quantum — v.57, v.60esr, v.68esr, v.78esr, and newer)

Instead of Lubuntu, consider BunsenLabs, which is based on Debian. BunsenLabs will run in a computer with 1 gigabyte of RAM, probably with plenty of swapping, so you'll still need at least 2 Gb of RAM. BunsenLabs LiveCD's actual commit after boot is ~200 Mb, which is equivalent to Windows XP SP3.

BunsenLabs is one of the few distros that offers a live CD (yes, a CD image), but the LiveCD edition is meant only for 32-bit systems.

As for Firefox, then off the top of my head:

In Settings:

• Disable "Check spelling as you type"

Under Performance:

• uncheck Use recommended settings;
• Play with "user hardware acceleretion"
• Set the Content process limit to 1

Under Browsing:

• Disable smooth scrolling
• Disable "recommend extensions as you browse"
• Disable "recommend features as you browse"

Under Search:

• Disable search suggestions
• Uncheck the search engines that you do not use
• Under Privacy & Security > Permissions:
  In Settings for Autoplay (new window), set the default for all sites to block audio and video. Use Ctrl+I, and in the Page Info window under the Permissions tab list for a domain, to allow audio/video on YouTube, for example.

Uncheck all under Firefox Data Collection. In about:config, set:

• browser.sessionhistory.max_entries to 25 (from 50)
• browser.sessionhistory.max_total_viewers to 0 (from -1)
• browser.sessionstore.max_tabs_undo to 5, 7, or 10 (from 25)
   
• browser.sessionstore.interval from 15000 *to* something greater
• extensions.update.interval from 86400 *to* something greater
• media.peerconnection.enabled to false
• media.peerconnection.video.enabled to false

Extensions:

• Install the NoScript extension, as it uses a whitelist (with some built-in entries). Set it to display full domains in settings. You'll have to permit certain domains from the NoScript pop-up menu to make a site work.
• Install uBlock Origin or Adblock Plus to block ads. While NoScript blocks most of the harmful cruft, one of the two adblockers won't let video ads pass through.