Debugging wins

After @ametaireau and I wasted way too much time fighting with debugging functional tests (and reproducing intermittent ones, here's a number of small improvements to our debugging environment which should help avoid this sort of wastage in the future:

* documents workaround to Firefox breakpoint-doesn't-fire bug
* enables and documents an easy way to run a single functional test repeatedly, with full logs, and drop into the Python debugger when it fails
* enables the Browser Toolbox in the functional testing profiles to make it significantly easier to inspect and debug the JavaScript side of a stopped functional test

Review / merging requested.