With the extension enabled, if I go to YouTube's homepage and then click the back button, it does not go back a page, however, the forward button lights up in Firefox's toolbar as if I did go back. I then have to hit back a second time to actually go back.
Here's a GIF that shows this issue in action: -
Steps I took: -
1. I went to Google and searched for YouTube.
2. I clicked on the link to YouTube and went to the site.
3. I clicked back - you can see in the GIF that it does not go back, but the forward button lights up as if it has.
4. I clicked back for a second time and this time it does go back.
If I disabled Adblock Plus, this does not happen. It also hasn't happened on any other sites apart from YouTube.