Many site issues can be caused by corrupt cookies or cache.

Warning ! ! This will log you out of sites you're logged in to. You may also lose any settings for that website.

• Delete cookies to remove the information websites have stored on your computer {web link}

• How to clear the Firefox cache {web link}

If there is still a problem, 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). Did this help?

While you are in safe mode;

Try disabling graphics hardware acceleration in Firefox. Since this feature was added to Firefox it has gradually improved but there are still a few glitches.

How to disable Hardware Acceleration {web link}

I cleaned the cookies for the the website and also all cache with no success.

Restarting in safe mode is what I previously did, also without success.

There are no animations in the site, how could this be related to graphics hardware?

I've used the site hundreds of times before without problems.

This seems a problem with Firefox. Does anyone from Mozilla care? If so, where should I report this?

https://support.mozilla.org/kb/Clear+Recent+History#w_how-do-i-remove-a-single-website-from-my-history

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 settings.

Before I lose more information, did you try to access this site?

It's not English so I can't sign-in.

This is just a username/password login page, but you shouldn't have to do anything to notice the problem with the performance. You should notice your CPU usage going high for just displaying that page.

You can also try to type a few characters in the username text box ("nome de utilizador") and you should observe it takes some seconds to display each character, because the browser is heavily loaded.

I used several versions of Firefox. No problem.

This is with version 67.0.

Also happens in a private window.

Have you tried in GNU/Linux?

Win 7

I notice the same happening. I can't use the Inspector or other developer tools to check what goes on because the debugger gets activated by some JS file that contains a debugger statement. The page responds extremely slow to key presses, so some heavy JavaScript is running (I can scroll with the mouse, but not with the keyboard).

Thanks, why is Chromium immune to this? Doesn't it run Javascript?

Do you run on GNU/Linux?

I reported this here: https://bugzilla.mozilla.org/show_bug.cgi?id=1557158