Standard advice is to (a) use "Safe" Mode to diagnose problems, and (b) use Refresh Firefox periodically to avoid incompatible extensions and other problems. Of course both "Safe" Mode and Refresh Firefox can be dangerous for those of us with strobe sensitivities, and can be unusable for those of us using accessibility extensions and other accessibility fixes. This article indicates that Refresh Firefox still isn't safe: https://support.mozilla.org/en-US/kb/refresh-firefox-reset-add-ons-and-settings "These items and settings will be removed: Extensions and themes, website permissions, modified preferences, added search engines, DOM storage, security certificate and device settings, download actions, plugin settings, toolbar customizations, user styles and social features will be removed." Note that blocking any significant proportion of animation requires installing extensions, modifying several preferences, and installing styles...