Firefox's use of CPU resources has improved but hasn't gone away completely.
The popular Web browser has had a longstanding CPU (Central Processing Unit) utilization issue that--in some cases--overtaxed the CPU, caused noticeable heat issues in small laptops.
I wrote about this last November after I had been grappling with this issue for more than a year. First, on an Hewlett-Packard business ultraportable and then on the Apple MacBook Air. As I stated at the time, my theory is that many users don't notice Firefox CPU utilization on large, well-ventilated mainstream laptops. But it can be an issue on ultraportables, which are more sensitive to heat because of the obvious design constraints (typically under an inch thick).
For more on this story, see Firefox 'CPU resources' issue better but not gone
on CNET News.