Enter about:config in the url bar, set gfx.webrender.all = false and restart the browser.

Better?

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 is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• 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

Hello, I have tried everything and the problem is still there. By disabling completely hardware acceleration, it gets smooth but with tearing right in the middle of the animating element. Funny how, 66.0.0 did not have that problem and the animation was super smooth even with hardware acceleration. I do not have the "3-bar" installed.

Did you check the webrender pref as I told you?

TyDraniu said

Did you check the webrender pref as I told you?

Yes, I did that too but with no effect.