After more than a year of using YSlow, apparently with the arrival of FireFox 3.5 this plug-in is no longer supported.
The plug-in, which used the FireBug extension, was a practical tool to analyze performance by the 14 rules described in the book, giving grades to pages at loading. A very useful tool, both for designers and front end developers.
But with the launch of FireFox 3.5, which brings many improvements in speed, YSlow becames incompatible. After what seemed like Yahoo! will totally give up the project a new version appeared. Unfortunately this version is not exactly functional. I do not know if is my FireBug version or just the update was made to shut the mouth of those who complain about the incompatibility.
PageSpeed structure is similar to YSlow, anyway optimization rules are the same for everybody, it’s just how they are approached.
What is more important PageSpeed is compatible with FireFox 3.5.
An interesting fact to know is that Steve Souders now works for Google, maybe this is one of the reasons for which these rival tools, produced by two Internet giants have reached these state.