First, I love this browser!!! On this latest update, the browser window is only about 85% of the total instead of 100% There is my background up from the bottom. This is the 1st time this has happened.
Try to rename/remove xulstore.json in the Firefox profile folder with Firefox closed.
You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.
- Help -> Troubleshooting Information -> Profile Folder/Directory:
Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder