A few more details -

I'm running a 13" model with an Iris Pro and Haswell 2.4ghz i5. No discrete graphics here.

The issue seems to be specifically with the Mac build. I can actually run Firefox inside of a Windows 10 or Ubuntu virtual machine (using VMware Fusion) and get BETTER performance than I do natively. The same link above works fine in a virtual machine. Very odd.

Note: V55 has been giving users problems. Reinstall v54.

https://support.mozilla.org/en-US/kb/install-older-version-of-firefox?cache=no Be sure to read everything here.

If you still want to downgrade, look under; I still want to downgrade. Click the Directory of other versions and languages link. Look for the directory of the version that you want. But, remember that old versions may have security issues.

You should also check out Firefox; Extended Support Release https://www.mozilla.org/en-US/firefox/organizations/ https://www.mozilla.org/en-US/firefox/organizations/notes/ https://www.mozilla.org/en-US/firefox/organizations/system-requirements/

Older versions have the same issue, unfortunately. I've been having the problem for over a year, but it's only recently that I'm wanting to give it another shot.

Reinstalled 54 with a brand new profile just to confirm it. Within 5 seconds of the video starting, my CPU was pegged and the entire machine slowed to a crawl.

I called for more help.

Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

https://support.mozilla.org/en-US/kb/firefox-slow-how-make-it-faster

https://support.mozilla.org/en-US/kb/firefox-uses-too-many-cpu-resources-how-fix

https://support.mozilla.org/en-US/kb/firefox-hangs-or-not-responding

Yep, problem persists in safe mode as well =/

Bump - any additional ideas? :(