Mozilla

Unfortunately in some browsers isn't such functionality built in therefore it is necessary to use some tricks or extensions. As well as it is necessary to differentiate between HTML5 and Flash video.

Chrome
HTML5
Such functionality will be built-in the Chrome since January 2018, now following can be used:
Disable HTML5 autoplay extension
https://chrome.google.com/webstore/detail/disable-html5-autoplay/efdhoaa...
Flash videos
In the Chrome menu, go to settings and scroll down to Show Advanced Settings. Click this and under the Privacy heading, click Content Settings. In the pop-up window, scroll down to Plug-ins and select the Click to play radio button.

Firefox
HTML5
(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
Flash videos
Flashstopper extension
https://addons.mozilla.org/cs/firefox/addon/flashstopper/

Opera
HTML5
Disable HTML5 Autoplay extension
https://addons.opera.com/en/extensions/details/disable-html5-autoplay/?d...
Flash videos
Use same approach as for Flash videos in Chrome (see above)