Maybe use an adblock? uBlock Origin is a good one. It's 2017, you shouldn't be needing to sit through ads anymore.

Sites may have found a way to start a video after page load that bypasses the autoplay.enabled = false setting. I'm sure there must be some loopholes. But first let's confirm what we're dealing with.

Could you right-click the video player and check the context menu? Plugin-based players tend to have short menus mentioning the name of the plugin (e.g., Flash, Silverlight), while the built-in HTML5 player tends to have either Firefox's long context menu with video-related commands at the top -- assuming the layout allows you to right-click directly on the video -- or a replacement menu determined by the site (e.g., on YouTube).

Moses, thanks for the referral. I'll have to mull over whether I want to install an app. I'm an IT minimalist.

jscher2000, it seems that the prevalence of the motion graphics depends on what machine I'm on. At the moment, the machine I'm on is my personal laptop -- I have a hosts file that seems to be successful in preventing access to many of the motion graphics. After a few tries, though, I received one that was not blocked. I've added an image of the context menu to this post. I'd appreciate your appraisal of the situation.

P.S. I know that some GIFs are motion graphics. Not sure of the above two measures would prevent them from downloading and "playing".

P.P.S. I got the laptop from which I posted the original post. No host file, more of the ads. It seems that the context menu depends on what ad is displayed. I captured the context menu for a bunch of different ads and updated the image below.

First, those all look like Firefox context menus. Two reflect a direct right-click on an image, perhaps an animated GIF, or perhaps a transparent layer in front of other content. None indicates a direct right-click on a video player.

Second, every single menu relates to content in an inline frame (<iframe>), which is a common method for embedding content hosted on another site, whether it's a YouTube video or an ad. If you expand This Frame, the View Frame Info dialog (Page Info for the framed page) would indicate the host site.

(Too bad the This Frame menu doesn't have Hide This Frame!)

I understand not trusting extensions to make decisions for you about what content is displayed and what is suppressed -- who creates these lists? I guess the question is at what point the burden of zapping them individually outweighs that concern.

You can possibly use Delete Node in the Inspector (context menu in left pane) to remove this node (iframe). I sometimes use this method in case of annoying content that stays fixed (i.e. doesn't scroll).

jscher2000, thanks for the explanation. For the personal laptop, over which I have full control, the ads aren't much of a problem. It's the work laptop that doesn't give me permissions to add things where it's more problematic. I was looking for a simple switch or setting that might address the problem, but it looks like the two I've already tried are the ones.

Thanks for hint about nodes, cor-el, but It's probably not practical for me to do this for each page that I visit. Since I'm not a developer, things that seem easy actually take a while for me to figure out using Google, but even if that were not the case, customizing each page I visit is a bit beyond what I'm prepared to do in order to avoid video ads. But I appreciate the insight.

For the work system, you aren't even able to install extensions from the Mozilla add-ons site? Maybe you could tell the boss you've heard people are more productive when they aren't distracted by ads, so could you get permission to install uBlock Origin...

Sorry, I should clarify. It is a locked down system, but even apart from what has been explicitly prevented via IT permissions, there are many rules so it's best to not try. I think I can say with confidence that things like this would not be a matter of discussion.