Add-ons

Session Manager by Michael Kraft

Session Manager saves and restores the state of all or some windows - either when you want it or automatically at startup and after crashes. It can also automatically save the state of open windows individually.

British English Dictionary by Mark Tyndall

British English Dictionary R1.19 for Firefox, Thunderbird and SeaMonkey by David Bartlett.

To enable, right-click in a multi-line text box, select "Languages", then "English / United Kingdom".

Mozilla Archive Format, with MHT and Faithful Save by Christopher Ottley, Paolo Amadini

— Open web pages saved with Internet Explorer or other browsers (MHT)
— Save many tabs, video and audio reliably, in a single ZIP file (MAFF)
— Return to the original site you saved a page from, and more!

Collector's Note The far superior way to save webpages.

Master Password+ by V@no

Various enhancements for build-in "master password" (MP)
-Locks master security device on demand or after specified time
-Prompts for MP on browser startup
-Suppresses all or duplicate MP prompts
-Locks up entire application or individual windows

Collector's Note Re-prompts you for your master password after a set period of time. Can also lock your Firefox session after a period of time.

Perspectives by perspectives-cmu, dschaefer

Connect securely to https websites by checking certificates with network notaries. See http://www.perspectives-project.org

Fragment Titles by John P Baker

Save better history for fragment URLs.