I think it is worth considering having umbrella coverage. In MMM's post on Insurance: A Tax on People Who Are Bad At Math?, MMM indicated that one instance to purchase insurance is when you cannot afford the consequence of a lack of insurance. Even if you do not currently have a large net worth, as a Mustacian, you hope to quickly build a large net worth. Finding yourself on the wrong end of a large personal injury judgment (as an example) would seriously hamper those plans. The judgment would follow you, attach to any real estate you own, and perhaps necessitate bankruptcy if you were unable so satisfy the judgment. And it is not hard for PI cases to greatly exceed a $100,000 homeowners liability limit. Finding a good rate on an umbrella policy is a relatively inexpensive way to protect your goal of FIRE. My personal calculation is that I cannot afford this consequence, as unlikely as it is.
Another valuable form of insurance often overlooked is the limit on your uninsured/underinsured motorist coverage in your auto policy. This is your hedge against being injured by another motorist not responsible enough to carry adequate insurance. This is one I consider worth paying a little extra.
Full disclosure: I am an attorney that works in a firm with a strong plaintiffs side personal injury practice group. I practice in business transactions and estate and business planning and don't do PI work, but observe my colleagues' cases. My perspective therefore maybe skewed to the risk-averse side.