You could try adding the following preference as a number in about:config. From memory, it is effective immediately but you may have to restart. It will mean you will see a barely motionless hourglass when loading a tab instead of the animated icon.

ui.prefersReducedMotion (give it the value 1)

https://support.mozilla.org/en-US/kb/about-config-editor-firefox

Sorry, I don't think I've explained it well enough. Perhaps this problem can only be solved with some javascript filter in AdBlocker or uBlock.

I meant -- is it possible to disable animations that the website itself creates via javascript (I think?), when the title of the tab (e.g. "Forum") changes (e.g., user receives a message, and the title starts flashing "New message! - Forum"). I was hoping there is some browser-wide configuration to disable such changes.

Here is an example of the discussion on how it can be integrated into a website

Sorry, I don't think I've explained it well enough. Perhaps this problem can only be solved with some javascript filter in AdBlocker or uBlock. I meant -- is it possible to disable animations that the website itself creates via javascript (I think?), when the title of the tab (e.g. "Forum") changes (e.g., user receives a message, and the title starts flashing "New message! - Forum"). I was hoping there is some browser-wide configuration to disable such changes. Here is an example of the discussion on how it can be integrated into a website

Hello! One way that I currently know of is by disabling the Javascript using Ublock Origin, click the icon marked below to do this:

It is done via this script, but I don't know where it is coming from as I do not see it in the Network Monitor.

You could try enabling Annoyance filters in uBlock Origin. I use AdGuard Annoyances and uBlock filters - Annoyances but others are listed.