The spacebar is a keyboard shortcut in modern browsers including Chrome and Firefox that can be used to jump incrementally down a page. Unless you hate your trackpad or mouse, it's unlikely you use this maneuver very often.
With the Chrome extension YouTube Pause, you can turn your spacebar into a play/pause button for YouTube. Simply install the extension and you'll be off and running; YouTube Pause does not require a restart of Chrome and does not include any settings.
A search for a similar add-on for Firefox came up empty, but YouTube Smart Pause automatically pauses YouTube videos when you switch tabs in Firefox or to another application.
In related news, YouTube is rolling out a new autoplay feature, and you have two ways to turn it off. Video creators may want to learn about YouTube's new interactive cards, while viewers should know these three ways to turn off YouTube's annotations and cards.