I am currently running Firefox 76.0.1 on Ubuntu 18.04 after a fresh install. I logged into my FF account and everything synced, bookmarks, add-ons, etc. But every time I restart the browser, even after a couple of seconds, my Matte Black theme changes to a theme that I have NEVER applied, made, nor even seen in the theme catalog, even on my previous Ubuntu installation. I tried running FF in Safe Mode and "Refreshed Firefox" but the problem occurs after a minute or two after starting the browser afterwards and the theme changes again. I deleted the profile .js file for a new file to be created, but that does not work. It's as if the Sync didn't sync the new changes made to the browser.