From MozillaZine Knowledge Base

You can find extensions for Mozilla applications at:

• Mozilla Add-ons
• The Add-ons Mirror (formerly the Extensions Mirror)
• Individual authors' sites:
  • Gorgias' Firefox Extensions
  • Chris Pederick's Mozilla Firefox Customizations
  • XUL Apps
• Extensions forum at MozillaZine: a good place to get a glimpse of new extensions being developed, or to try out new extensions before they become available at the sites listed above.

See Installing extensions for guidance.