Description of problem:

When I click on a file to download, e.g. a Fedora installation image ISO, Firefox CPU load goes from the 10-20% range to 80+% topping out at 150% until the download is finished and then it goes back to 10-20%.

Version-Release number of selected component (if applicable):
firefox-45.0.2-1.fc23.x86_64


How reproducible:
Always, does not appear to be a regression


Steps to Reproduce:
1. Run top
2. Launch Firefox, note CPU usage
3. Start a file download, note CPU usage
4. Wait for it to finish or cancel the download, note CPU usage

Actual results:

Huge increase in CPU consumption only when downloading.

Expected results:

This shouldn't use much more CPU than using curl or wget.

Additional info:

I get the same results with a Mozilla tarball of Firefox.

I'm not sure what information to include to track this down. 'strace -p -ff -o' or maybe attach gdb, but I'm not sure exactly what to do there at all, but if I get some guidance I'll provide what I can.