I found that Firefox 3.5.3 has been very slow due to the Firebug plugin. And I found out how to fix the problem.
As a web developer who builds web sites using php Content Management Systems like Joomla and WordPress and shopping carts like X-Cart, I find the Firefox Firebug plugin invaluable. It makes the modification of css much easier and faster; it allows you to quickly determine how the CSS is applied to specific parts of a web page. Unfortunately, the Firebug plugin also makes Firefox extremely slow, especially when running complex scripts/programs like WordPress, Joomla, X-Cart and any other web applications that use a lot of Java Script.
For example, if you turn on Firebug, it is turned on for all tabs in your browser. As a result, if you are running Joomla Administrator or WordPress administrator, Firefox will be extremely slow and sometimes crash or hang due to the Firebug plugin. There is an easy way to fix the problem. Turn off, or Deactivate Firebug for any page on which you don’t need it. Turning Firebug off can be a little confusing since you can toggle the Firebug panel on or off, but toggling Firebug does not turn it off.