From MozillaZine Knowledge Base
The following browser extensions may be helpful for some users. They are not part of Firefox; you will need to install them separately. If you have a problem with a browser extension, please contact the extension author.
- Mozilla/Firefox Accessibility Extension - View and navigate web content based on structural markup.
- Menu Editor - Rearrange menu items. Hide unused menu items. Add/Remove menu separators.
- Page Zoom Buttons , toolbar buttons to zoom in, zoom out, and reset a web page back to its default view instantly from a single toolbar button (if desired) by clicking on, scrolling over the button, or both.
- Organize Status Bar rearrange or remove any item (icon or text) in the Firefox status bar
- keyconfig - Assign keyboard shortcuts to any command or menu item.
- All-in-One Gestures - Execute common commands using mouse gestures, rocker navigation, scroll wheel navigation and page scrolling
- Grab and Drag Adobe and Mac like scrolling, Flick gestures, and Momentum scrolling useful for Tablet PCs, UMPCs, and other pen or touchscreen devices. 
- FlashBlock - Prevent Flash content from loading until you click it.
- Tab Mix Plus - Fine-tune tabbed browsing. (may conflict with many other extensions/settings)
- PopupSound - Play a sound when a popup window is blocked.