Firecookie is an extension to Firebug that allows managing cookies in your browser. Latest 1.0v has been also just released on AMO and it's fully compatible with Firebug 1.5.
Break On Cookie Change
To activate the feature, select the Cookies panel and click on Break On Cookie Change button.
As soon as a cookie on the current page is modified, Firebug debugger halts and the Script panel is activated.
If the developer is interested only in one specific cookie, a cookie breakpoint can be created by clicking on the breakpoint column, just like in case of a regular JS breakpoint.
So, if this particular cookie is modified the debugger stops JS execution showing what line of source code is the culprit.
In order to break only under some circumstances, a breakpoint condition can be specified. Just right click on the breakpoint to show a condition editor.
The breakpoint will stop only if the expression is evaluated to true. Note that you can use following cookie properties in the expression (as built in variables): value, path, host, expires [ms], isHttpOnly, isDomain.