Firefox 4 (betas) only: If you dig bleeding-edge Mozilla experiments like the quick-launching Ubiquity, uber-organiser Contacts or single site app engine Prism, you can keep them all installed and updated with Lab Kit. It's a meta-extension for Firefox that provides Chrome-like Labs options.
With Lab Kit installed, you get links to all the Mozilla Labs experiments, but you also get quick installation and automatic updates to a batch of extensions — Test Pilot, Prospector – Speak Words, and Contacts — that Mozilla has already included with the Kit. Future extensions are sure to be rolled in soon, and you can avoid clicking through multiple extension installers to try out the cool stuff that could be coming soon to Firefox.
Lab Kit is a free download for Firefox 4, currently available in beta form for Windows, Mac and Linux systems.