I found a solution that works for me!
When you generate the embed code on YouTube.com, you can select “Use privacy-enhanced mode.” This changes the link within the embed code from “https://www.youtube.com/embed/…” to “https://www.youtube-nocookie.com/embed/”
Using this new code on my WordPress site, I no longer have to hit the back button twice, and if I right-click on the back button I just see one instance of each page, as should be the case.
I realize now that this question is more related to Firefox then to WordPress, but I include this solution in case it helps anyone.