Conditional breakpoints are useful for adding breakpoints to the code without having to hit them every time. For example, if you know there is a bug in your code at the last item you can use a conditional comment to check whether it is the last item before activating the debugger. The breakpoint will fire whenever the condition is TRUE.
To create a conditional breakpoint in Chrome, create a normal breakpoint and then right click on the breakpoint and select "Edit Breakpoint" to add a condition.
In Firebug, conditional breakpoints can be created by first creating a normal breakpoint and then right clicking. A input box will appear allowing you to add a condition.
In Internet Explorer, create a normal breakpoint. Right click on the breakpoint and select "Condition...".
In Opera, create a normal breakpoint and then right click on the breakpoint before selecting "Add Condition".
In Safari, ensure both the script and "Debugger" panel are open. Create a breakpoint by selecting the line of code you wish to debug. Then in the "Debugger" panel, right click on the same breakpoint and choose "Edit Breakpoint".