It's unlikely. Extensions (like ad blockers) generally have a very low impact on performance in Firefox. Additionally, most of the ad blocking work is done when the page loads, not all of the time.

Note that videos are usually requested as a lot of small chunks and this causes constantly sending requests to the server. An Adblocker needs to intercept and check these requests in order to block possible ads or other unwanted content. The impact of this checking depends on how many filter rules need to be checked.

This is different than a static page that doesn't send requests once the page has loaded.