Love the add in but all of a sudden I can't add new sites to subscription groups in Firefox. I am able to add the groups in Chrome though. Any ideas?

Update 7/8/18:
On 6/13 I sent the developer a couple of screenshots because he requested them in a private email he sent me on 6/11. I am assuming that he received the email. I have not heard anything further from the developer regarding the issue I presented. I am still hopeful that there is a resolution to my problem. I have removed another star from my review, I will be glad to give 5 stars once the issue I am experiencing is resolved.

Update 7/25/18:
Since I still haven't received a response from the developer I uninstalled the add=on and then reinstalled it. This did not correct the original issue. I noticed that even though I uninstalled the add-on the subscription groups I created were still present after I reinstalled it. I wonder if there may be a limitation to how many subscription groups or subscriptions you can have in Firefox. As I originally stated I do not have this issue in Chrome.

Used to work great, now I get errors from Firefox that this script in particular is slowing down the page. I have to temporarily disable it to watch a video or even get a channel's homepage to YouTube to load. It even does this if I have the side panel closed. I have 564 subscriptions and I am using a 64-bit version of Firefox on Windows 10 Professional. Not sure what's going on beyond that. Very sad, though.