My brokerage firm sold some stock, but I think they used the incorrect cost basis information (was incorrect when I transferred the stock). Is this easy to adjust?

Yes, it is OK and it is easy to adjust incorrect cost basis. First, please report your stock basis exactly as it appears on the form 1099-B, then select  I'll enter additional information on my own feature and simply enter the correct cost basis. That way, you match the IRS reporting with the correct basis. You don't have to do anything else on your tax return.