You have to combine a number of approaches because sites use different methods for playing media:

• Plugin-based players: many sites still use Flash for videos, and some may use Silverlight, QuickTime, or VLC. You can set yours media-related plugins to "click to play" ("Ask to Activate") to control when they start. More on this below.

• Built-in HTML5 video player: Firefox has a setting to stop auto-play. On some sites there is a strange side effect where Firefox starts buffering the video and blocks playback without changing the pause button back into a play button. So on those sites, when you do want to start the video, you have to click twice.

And now the details:

To set "Ask to Activate", open the Add-ons page using either:

• Ctrl+Shift+a
• "3-bar" menu button (or Tools menu) > Add-ons

In the left column, click Plugins. Look for "Shockwave Flash" and change "Always Activate" to "Ask to Activate". Other media plugins usually are already set to "Ask to Activate" but you can double-check.

With this setting, when you visit a site that wants to use Flash (etc.), you should see a notification icon in the address bar and usually (but not always) one of the following: a link in a black rectangle in the page or an infobar sliding down between the toolbar area and the page.

The plugin notification icon in the address bar typically looks like a small, dark gray Lego block. When the page wants to use a blocked plugin, the icon turns red to alert you to the concern.

If you are ready to play the video and the site looks trustworthy, you can go ahead and click the notification icon in the address bar to allow Flash. You can trust the site for the time being or permanently.

Some pages use Flash only for tracking or playing ads, so if you don't see an immediate need for Flash, feel free to ignore the notification! It will just sit there in case you want to use it later.

If you want finer control: This extension lets you allow specific videos of interest to you while not allowing the rest of them: https://addons.mozilla.org/firefox/addon/click-to-play-per-element/ (I haven't tried it recently)

To control the built-in HTML5 player:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful.

(2) In the search box above the list, type or paste media*play and pause while the list is filtered

(3) To delay media from auto-starting in background tabs, I suggest: double-click the media.block-play-until-visible preference to switch the value from false to true

(4) To prevent media from auto-starting in any tabs: double-click the media.autoplay.enabled preference to switch the value from true to false

Note that setting media.autoplay.enabled to false may cause problems on some websites like not being able to start playing the video.

This worked:

To prevent media from auto-starting in any tabs: double-click the media.autoplay.enabled preference to switch the value from true to false

Thank you for that information. It is extremely rude for website designers to assume that everyone wants to see and hear their stupid obnoxious videos.

I think that Firefox should set this to disable by default. If there is anyone strange enough who wants to be assaulted by every website's obnoxious video, they can set it to enable.

Also, Firefox should make this an easy-to-find checkbox in the Preferences. Having to search the Help and run about:config to set this is too difficult for the average browser user.