This only happens on that one site, nowhere else?

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/websites-look-wrong-or-appear-differently

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

https://support.mozilla.org/kb/Firefox+is+already+running+but+is+not+responding

Started in Safe mode as suggested but no improvement. Seems only to be affecting on the Apple Website

Remove History For One Site Open the History Manager <Control><Shift> H. In the search bar, enter the name of the site. Right click on one of the listings and select Forget About This Site. This should remove all information, including any passwords / settings.

FredMcD said

Remove History For One Site Open the History Manager <Control><Shift> H. In the search bar, enter the name of the site. Right click on one of the listings and select Forget About This Site. This should remove all information, including any passwords / settings.

I have this problem - I'vetried the previous fixes but it doesn't solve the problem - this only happens with the Apple ID login page and I have to use MS Edge to access my Apple account - I am using Windows 10 and LastPass to login

LastPass has caused issues in the past. Update / disable LastPass.

Disabled LastPass and FF still hangs and memory usage increases rapidly as before

I called for more help.