It's mostly when clicking different folders like "spam." I wasn't sure what was going as this just started happening the past 3 days when I noticed that sure enough, I've been moved over to Gmail 2.0. I tried disabling and then finally uninstalling all my add-ons (plugins) to Firefox to no avail. Selecting "older version" in the top right nav of the gmail page made the problem go away. If you're listening google, please fix this.
UPDATE: you may need to select "older version" every day or at least a couple of days a week since gmail doesn't seem to remember that setting. And even the latest Firefox 188.8.131.52 with no extensions has this problem.