aquatic-skeletons:

If you have firefox just go into the configuration menu by typing about:config in the URL bar, tap your [enter] key to bypass the warning.

Type in: media.autoplay.enabled , right click and then tap [Toggle] to change it from [true] to [false] when it shows up.

Then type: plugins.click_to_play , right click, and then tap [toggle] when it shows up. Though this one has shown limited to no success, it’s worth trying anyway.

As for Chrome. It’s a bit harder to do the same as above since Chrome lacks the above configuration styles.

For chrome, if you want to successfully block tumblr’s Autoplay system. Which seems to utilise html5 last I checked. You will need to install [Disable HTML5 autoplay]. It works good for its intended purpose.

Edit:

If you’re having trouble with the above methods or do not like
disabling flash or enabling click-to-play on your browser, try using the
new Xkit exstension.

New X-kit Blog
Github Download (Firefox xpi)
Chrome Store Download

After
installing, tap the new icon placed in your header, it will open up a
in-browser popup. Tap ‘Get Extensions’ and then scroll down to ‘Vanilla Videos’
and tap the green ‘Install’ button. It will install after a second or
so. Vanilla Videos stops the player from following you down the page,
looping, and autoplaying. This only works on Tumblr whereas the other
methods work for the entire browser.

It’s also come to my
attention that Firefox (with the newest updates) keeps blocking and
disabling Xkit because it isn’t ‘Signed’. Gosh, that’s annoying. To get
rid of that Go to about:config tap [enter] to bypass the warning, type in, then [toggle] ‘ xpinstall.signatures.required ‘ to [False] and restart the browser.