Addons for testing Web page performance as well as try this

Page Speed, an open source Firefox/Firebug add-on that evaluates the performance of web pages and gives suggestions for improvement.

YSlow, a free tool from Yahoo! that suggests ways to improve website speed.

WebPagetest shows a waterfall view of your pages’ load performance plus an optimization checklist.

In Webmaster Tools, Labs > Site Performance shows the speed of your website as experienced by users around the world as in the chart below. 

Still the above mentioned items are in progress....



Chrome
Awesome Screenshot: Capture & Annotate
Edit This Cookie 0.31
Eye Dropper 0.2.6
Firebug Lite for Google Chrome™ 1.
Frame two pages 1.6.1
Google Dictionary (by Google) 3.0.15
IP Address and Domain Information 3.11
MeasureIt 0.4.9
Page Ruler 0.1.4
PerfectPixel by WellDoneCode 1.14
Speed Dial 2.1
Speed Tracer (by Google) 0.26
Web Developer 0.4.1
Websecurify 1.1.3

Firefox
DownThemAll
Extended Status bar
Selenium IDE
Firebug
Firefinder
Firepicker
Firerainbow
Flashfirebug
Groundspeed
HttpFox
Httprequester
Measure IT
Marklogic console for firebug
Page hacker
Pixel perfect
Regular Expression tester
Rest Client
Spell Checker
SQl Inject me
Xss Me
Auto pager
controlopacity
GreaseMonkey
Hackbar
PDF download
REload Every
Save as PDF
screengrab
Seo Quake
ShowLocation
ShowIP
Web2PDF converter
Download helper