Very similar issue! My gf walked into it recently on ArchLinux, she had the same problem, for her e10 was enabled by default. When she visisted pons.com the who

Very similar issue! My gf walked into it recently on ArchLinux, she had the same problem, for her e10 was enabled by default. When she visisted pons.com the whole browser was frozen for at least 10seconds, and if it didn't unfreeze itself after that time, it was just consuming memory and freezing whole OS until Linux killed Firefox process. My issue was solved by disabling e10s, for her it didn't help so she switched to Opera already.