Let me guess: had he by any chance installed a metric buttload of extensions? And then not tried creating a new profile in order to isolate whatever was soaking up the CPU cycles? Because that's usually the cause of Firefox performance problems ...
Actually the frequent updates to Firefox break the compatibility of so many extensions that it is rather difficult to run a buttload of extensions.
I had been running the current version of Firefox until an hour before I visited the site running Chrome. Bad assumption from using one piece of information.