Hidden but useful mozilla preferences

To access full list of Mozilla preferences type about:config in the URL bar. Among them there is a number of useful settings inaccessible otherwise.

wallet.crypto.autocompleteoverride: true
Allow to store password even for sites which intensionally opted out Password Manager (e.g., Yahoo) see related Bug 93776 and Bug 124065

for older versions: browser.block.target_new_window: true
for newer versions:
browser.link.open_external: 3
browser.link.open_newwindow: 1
browser.link.open_newwindow.ui: 1
Disallow browser to open any new windows by site request. Very useful for fans of tabbed browsing. See Bug 172962.

mail.compose.wrap_to_window_width: true
mail.fixed_width_messages: false
Wraps mail messages according to your current MailNews window size. Eliminates need of the horizontal scrollbar.
mail.autoComplete.highlightNonMatches: false
Disable @localdomain autocompletion. See Bug 93453.

network.sendRefererHeader: 0, 1, or 2 For users concerned about their privacy. Sending http referrers reveals browsing habits and allow to trace the browsing back. See Bug 55477.