When a page contains the <video src="some.webm"> tag, it can easily set loop to true.

However, when media is opened in a tab directly in the "stand-alone player", it is governed by Firefox's default HTML. I'm not aware of any available setting to turn on looping in this context. Someone else might have an idea, or perhaps there is an add-on to do it.

As a workaround, what do you think about a bookmarklet? You would click this to turn on looping (before the video ends). Here's how to test it out:

(1) Select and copy the script (it's all one line)

javascript:var vids=document.getElementsByTagName("video"); for (i=0; i<vids.length; i++) vids[i].setAttribute("loop", "true"); void 0;

(2) Right-click your Bookmarks Toolbar and choose New Bookmark

If you do not normally display the Bookmarks Toolbar, then you can create it on the Bookmarks Menu instead. You can create it somewhere on the menu in the Library dialog (Ctrl+Shift+b) and then drag it to the preferred position.

(3) Paste the script into the Location field

(4) Type the name of your choice into the Name field, for example, LoopVids

(5) Click Add to save it

To run the script on a page, click the button. That's it.

That works, thanks. I'd rather have some setting i could set once, but this works too.

I wanted Webm files to loop as well and I created a userscript for this. You can find it here: https://github.com/WhatIsThisImNotGoodWithComputers/webm-looper-userscript