Hi. Thanks for the review. Seems that you're not the only one that has this problem. Sadly I don't think there's a solution for it because it doesn't seem to be a problem created by the code from the addon but most probably after a Firefox Update.
See this thread for more info: https://github.com/ReDEnergy/SessionSync/issues/103

As much as I hate the issue, I can't really implement a solution for it because there is nothing wrong with the addon itself. If you try and open the addon view inside the Windows Mode or the Tab Mode you will see that the scrolling works just fine. The issue is related to the Firefox profile you have I believe. Sadly it seems that Firefox updates constantly keep corrupting (for various people) the profile. This might also be a problem created by the same issue. I understand your frustration but there's literately nothing I can do, if the same code works in Tab Mode / Window mode (CTRL + SHIFT + V or CTRL + SHIFT + U) then it means that Firefox somehow broke the Panel mode (and it's not reporting right the information inside). If you would install the addon on a clean profile you will see that everything works as expected.

Thanks for point it out, but unless I can somehow find the issue and investigate more about it, there's nothing I can do. Again, looks like is not a problem of the addon, but I might be able to somehow do things differently to avoid this Issue I will try.