My current list of Firefox Extensions and Add-ons. After multiple performance and stability issues, I am being far less trigger-happy in installing these so this pared-down list is what I am using at the moment.
AdBlock Plus + Filterset.G Updater + Element Hiding Helper – safer surfing
All-in-One Gestures – highly touted but I haven’t honestly had the time to really mess with it
All-in-One Sidebar – Nice add-on for getting better visibility on various Firefox things like history, bookmarks, extensions, etc.
Auto Dial – Nice add-on for Ctrl/Cmd+T access to commonly accessed sites (thanks to ShriNagesh!)
Download Statusbar – easy way to check on how downloads are progressing
Firebug + YSlow – method for checking on performance issues with webpages. (NOTE: On Internet Explorer the equivalent is HTTPWatch)
Foxmarks – keep bookmarks backed-up and consistent across multiple platforms
IE Tab (Windows only) – for avoiding use of IE as much as possible
Google Gears – for off-line management of Remember-The-Milk, Google Calendar and not enough other apps
Google Gears – for off-line access to Google Apps (hopefully they’ll get mail up here soon!)
Google Notebook – great extension for editing Notebooks fast
Google Preview – get a preview of the search results before the click
Greasemonkey – scripting foundation for lots of the Firefox plugins – lots of great scripts for GMail such as Remember The Milk for Gmail and GTDinbox
Live HTTP Headers – for more URL analysis (thanks to the comment registered below!)
ScribeFire – Quick blog-posting software
Stylish – thanks to this Lifehacker article, this one tunes the interface of Firefox – I especially like setting the address bar in yellow when using https sites
TagSifter – for getting a quick cloud of all your bookmark tags
TinyURL Creator – a Java script for the Bookmark Bar to quickly get tinyurl’s generated for posting to blog entries, twitter, etc. Also works on iPhone.
Notes: slowdowns on Leopard 10.5.4 with latest patches: use latest Flash plugin and try new profile
Profiles quite useful for swapping proxy configurations