I just went through a bad experience trying to merge multiple old address books, including several abook-1.mab files after a hard drive crash and I came up with this solution. My thought is to make any .mab file importable by renaming it to abook.mab, opening Thunderbird with it in the profile, and exporting it. This appears to work in Win7 with TB 45. I apologize if this is detailed somewhere else, I didn't find it.