If you're using Firefox 52 or later and need to switch back to the old Debugger, you can do so by visiting about:config and setting the "devtools.debugger.new-debugger-frontend" preference to
A normal breakpoint is just associated with a line: when the program reaches that line, the debugger pauses. A conditional breakpoint also has a condition associated with it, which is represented as an expression. When the program reaches the line, the debugger pauses only if the expression evaluates to
To set a conditional breakpoint, activate the context menu in the source pane, on the line where you want the breakpoint, and select "Add Conditional Breakpoint". You'll then see a textbox where you can enter the expression. Press Return to finish.
Conditional breakpoints are shown as orange arrows laid over the line number.
If you context-click on any breakpoint, you'll see a menu item "Edit Breakpoint". You can use this to modify an existing condition or to add a condition to a normal breakpoint.