Thanks for the swift response. These days it is hard to see how web sites can be developed without Firebug and YSlow is an excellent add-on to Firefox. Some of the other tools in the list I was not aware of and will check them. Some of those remaining tools have not been maintained for a while, such as httperf.

I have also extensively checked the listings at opensourcetesting.org, but the vast majority is either ill-maintained, dependent on Java/Python or Windows or only measure responses for stress tests.