Checking into it further I see I also have a problem with multiple numbered sessionstore.js files in the FireFox profile folder dating from when I updated from FF3.5 to FF12.0. The multiple numbered pattern.inis started, I think, when I updated ABP from v2.0.3 to v2.1.2 after updating to FF15.0.1.
I found a post by someone who also had the problem with multiple numbered session.js files and guessed that the problem was caused by his anti-virus on-access scanning.
I assume Antivir "locks" .js files upon access, in order to scan them, causing FF to create a new sessionstore.js file.
So I made an exception to my ant-virus on-access scanning for the FireFox "profile" and "adblockplus" folders. This seems to have put an end to the creation of multiple numbered session.js and pattern.ini files in their respective folders.
I have a version Eset NOD32 as my anti-virus that hasn't had it's virus definitions updated since 8/11/11, which is more than 10 months before I updated from FF3.5, so I don't think it's that the files have suddenly started being identified as a problem by NOD32. My computer is old without a lot of memory, which may be the main factor in this problem. I'm running a Northwood 2.4B with 1.5GB of memory. I also tend to keep a dozen or two or three tabs open in FireFox at a time - which uses a lot of memory. Always have Skype and OrbitDownloader running with Comodo and NOD32 as well.
It's only been about 12 hours since I made the changes to NOD32 - I'll update if the problem rears up again, or in several days if it hasn't.