They are extensions of Firebug which is, as you may know, a (well-known) plugin for Firefox. In other words, they are plugins of a plugin for Firefox.

YSlow (with Firefox 3 support)

Based on best practices for high performance web sites (a research conducted by the Exceptional Performance team of Yahoo), YSlow analyzes web pages and tells you why they’re slow.

The plugin adds a new tab to Firebug console and do analyses on the page on opening. Though being criticized for some impractical rules like using content delivery network (CDN), it’s very useful tool for those who want to have general view of web app performance from client side. Following rules where you can (but don’t care much about grades it gives) you will see your web apps running faster while eating less bandwidth.

Image: YSlow in action

The team has released a new version of YSlow that works with Firefox 3. The new version includes:

• Firefox 3 and Firebug 1.2 beta support
• improved and simplified check for javascript minification
• different coloring for inline vs. external CSS and JS (”All CSS” and “All JS” features)
• clickable list of resources as a Table of Contents (”All CSS” and “All JS” features)
• improved colors and presentation in the “legend” of component pies under Stats
• fixed a bug where the same hostname with different port number was counted as a separate DNS lookup
• misc bugfixes and style tweaks

This version supports all possible combinations of the different Firefox and Firebug development branches, namely the latest Firefox 2 and 3 and the latest Firebug releases: 1.05 (stable), 1.1 (beta) and 1.2 (beta).

You can install it from the YSlow page or the Mozilla add-ons site.

Jiffy

The Jiffy Firefox Extension is written Bill Scott of Netflix. Like YSlow, it adds an additional panel to Firebug that provides a visual view of the JavaScript time measurements captured by Jiffy-Web.

You can install it from this Mozilla add-on page (login required) and find more info about how it works here and here.

It’s also compatible with Firefox 3 and the latest Firebug beta (Firebug 1.2.0b3).