(Many from here. Full manual here)

browser.search.openintab (overridden by Tab Mix Plus)

• boolean - open stuff from the search box in a new tab?

browser.tabs.closeButtons (overridden by Tab Mix Plus)

• Modified Value: 3 (revert to Firefox 1.5 behavior)
• Alternate Modified Value: 2 (don't display any close tab buttons)
• Default: 1 (display close buttons on all tabs)

network.prefetch-next

• Modified Value: false (kill speculative prefetching)

Layout.spellcheckDefault

• 0 - Disable spellchecker.
• 1 - Enable spellchecker for multi-line controls (e.g., <textarea>s). (Default in Firefox and SeaMonkey)
• 2 - Enable spellchecker for multi-line controls and single-line controls. (Default in Camino)

security.dialog_enable_delay

• Set to 0 to disable the dialog delay