What sort of website supports specific versions of Firefox and doesn't support other versions?

URL of that website, please?

Did you check your Avast security software to make sure that this isn't causing your slowness issues?

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Appearance
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Boot the computer in Windows Safe Mode with network support (press F8 on the boot screen or hold down the Shift key) to see if that has effect.

• http://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/
• http://www.7tutorials.com/4-ways-boot-safe-mode-windows-10

The website is for catchup on itvplayer. E.g. https://www.itv.com/hub/the-loch/2a3969a0006. It looks like I'm not the only one getting slow responses since moving to 52.2.1 ESR. I'll have to see if they got a solution.

You have Video issues. There are errors in the info that was sent. Video issues will cause things to be slow.

Both AMD & Nvidia released drivers this week. Please re-install your drivers or update your drivers. https://support.mozilla.org/en-US/kb/upgrade-graphics-drivers-use-hardware-acceleration ALSO TRY NO HARDWARE ACCELERATION

Try these : 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

Please let us know if this solved your issue or if need further assistance.

For some unknown reason Firefox speeded up before I updated the Nvidia driver. Stll cannot use catchup on itvplayer.