My parents house (in the EU) was covered in plates of asbestos. As far as I know it only is dangerous when you install / remove it as these plates break and release chemicals to the air. I’m no expert though.
As older Firefox versions are still in use, too, it’s possible to check whether the improvements come from updates in Firefox itself or the web in general.
I can recommend Kirby (https://getkirby.com/), a flat file PHP CMS. It’s fast, has a panel to update data and can be hosted on any basically any PHP host. Just use the quite simple PHP-templates and add CSS & JS like you already know how to do. No need to complicate things.