It sounds to me like Firefox is attempting to load the page before the addon(blocker) is actually loaded. Do you get the same result regardless of the browser running beforehand or not?

I just closed Firefox and then clicked on the link in the email. It started up Firefox with a new tab for the YouTube link to run in. The ad appeared at the start like usual. So having Firefox closed gives the same result as having Firefox running when I click on the link.

Thanks for asking!

It seems like Thunderbird does something in the background that shuts off the adblocker when it starts the tab to run the YouTube video.

I noticed that when I click on another YouTube video on the side of the window an ad appears when that link begins. I clicked on a couple other videos and sometimes the appears and other times it doesn't. Is this because the user ID that created the video allows or disallows ads? I have no idea. However I can say with certainty that when I start YouTube myself in an existing tab the ads never appear. When I leave YouTube and go to a different website the adblocker still works. I changed to USAToday and it asked me to turn off my adblocker so it knows the adblocker is running.

My best guess is that Thunderbird does something that allows YouTube to ignore the adblocker when Firefox opens a new tab for YouTube. Whatever text they have in the programming that starts up Firefox or opens a new tab might have an command option that tells YouTube to ignore the adblocker. It would be interesting to see the program that Thunderbird executes when it starts up YouTube in Firefox. Is this behavior unique to Firefox or would it happen if I was using another browser as my default browser? If it's independent of the browser that's being used then it must be something that is done when calling up YouTube. If it only happens with Firefox then it's something that happens when Thunderbird calls up Firefox. This is just a guess on my part but it seems like this is a reasonable way to look at it. I'm not going to change my default browser to something else at this time because I don't want to spend time playing around with it. I might try this at a later date.

I'm not a programmer so I'd need to do some basic self educating if I want to pursue this on my own. That's why I thought I'd ask about it here and see if there's something that someone in the user community might know about this.

Thanks again for asking about this!