UserAgent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:60.0) Gecko/20100101 Firefox/60.0

Steps to reproduce the problem:
1. Open developer tools
2. Select network tab
3. Visit http://game.granbluefantasy.jp/#top
4. Right click anything in the network tab and pick 'Save as HAR with content'

What is the expected behavior?
It should work

What went wrong?
The devtools remain in 'collecting content...' state forever, the progress bar never fills up, the red 'stop' square doesn't do anything, and the .har file is zero bytes

Did this work before? Yes Haven't used it in a while but it definitely used to work

Chrome version: 65.0.3325.181 (Official Build) (64-bit) (cohort: Stable)  Channel: stable
OS Version: 10.0
Flash Version: 

This happens anywhere on this website, maybe because it uses dynamic content? I can save 'www.google.com' successfully and it's basically instant