Quite happy with FF Nightly and the new WebRender Experimental setting, seems to be faster. Dunno if it helps here but might help to see if it's fixed in up-coming versions or not.
The app does some reason in unresponsive on first start after an update. I had to restart the app after the app and it starts up fine.
For the next update, I will see if this happens again. If that's the case, I will fix up the package to restart the app. I couldn't find anything in the release notes