Rule of thumb: updating an application is rarely a solution to an issue that suddenly comes up, so it’s best to fix that first. Is the issue similar in 54 and 55? As you wrote "as of today", you should wonder what happened between yesterday and today, so try to check logs. Did you receive any Windows updates for instance, or for an add-on? Does the issue occur when starting Firefox in [[Troubleshoot Firefox issues using Safe Mode|Safe Mode]]? (Note: do not attempt to downgrade to 54, or you may run into issues with regard to migrated profile data.) When unclear, I would first uninstall Flash when Firefox is closed, and possibly reboot. This is to rule out that Flash interferes - basically you don’t need Flash for YourTube videos at all, as they default to HTML5. You can check your machine’s playback capablities [https://www.youtube.com/html5 here]. If that does not work and hence you are sure non-Flash content is an issue too, create a new profile as instructed in the KB’s [[Use the Profile Manager to create and remove Firefox profiles|Profile Manager]] article, and launch it. A default profile should be created and in use (which is always good for troubleshooting), and if things work, that would indicate there is something wrong in your current profile, and we could continue from there. Also, be sure to [[Upgrade your graphics drivers to use hardware acceleration and WebGL|update your graphics drivers]] when possible. You may also just need to disable ''hardware acceleration'' as suggested in that article, though that would indicate something changed in that area compared to yesterday if the issue is new.

Just in case the above doesn't do the trick, see : https://support.mozilla.org/en-US/questions/1174574