I'm not sure whether it is part of the group of behaviors controlled by the following preference. Does it help to toggle it:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

(2) In the search box above the list, type or paste cosm and pause while the list is filtered

(3) Double-click the toolkit.cosmeticAnimations.enabled preference to switch the value from true to false

I checked toolkit.cosmeticAnimations.enabled It was already set to false.

tabs take almost one second for open/close. Someone probably thought it looks nice but didn't think it would slow fast users down.

I'm not even sure there is an about:config to disable it.

Are you sure that this is an animation and not some other issue ?

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 -> 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

c1460286 said

I'm not even sure there is an about:config to disable it.

Did you check the value of the preference with Firefox version 73.0, or with Firefox version 68.0 (according to your system details that's the version you posted this with) ?

I'm asking because of this "askvg" article (especially the last two comments):

https://www.askvg.com/tip-make-mozilla-firefox-faster-by-disabling-ui-animations/