I am sure this is a duplicate of another SO question, as I feel like I answered it earlier, but I cannot find it. If I subsequently find it, I will update my answer.
You need to set the prefix extensions.lastAppVersionaccording to the version of Firefox that you are running. Otherwise, you will usually have an additional tab in Firefox and the Addons pop-up menu.
You may need to configure Selenium to use a custom Firefox profile to make this change.
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