Use the free version, update weekly and run scan. Downloaded database v 4298, and started quick scan. After 2m31s, MWB froze with this file showing -
c:\program files\mozilla firefox\plugins\npsabffx.dll. Had to reboot to resume. Checked with MWB to ensure I had the latest database, then tried scan again - same time, same file and froze again.

I have learned that this particular file is installed by SuperAdBlocker and it must have been in my FF plugins for some considerable time. After reading advice on Mozilla, I renamed the file, and ran MWB again, this time successfully. I can think of nothing which has changed on my PC since the previous scan a week ago, other than the updated MWB database.

Anyone had a similar problem? I have posted on MWB's forum, but no replies so far. Haven't noticed any problems with ads since renaming the file, so it's not a serious glitch, so far ....