I'm getting a lot of these in the past 2 weeks. I scanned for adware etc. with Iobit and Malwarebytes. those and avast all came up clean.
I'm using FF 25.0.1 in W7 pro 64bit. The only thing that stops them is disabling Flash player, but Flash is needed for Youtube etc., games and some uploaders on hosting sites I use.
Most of these ads are for dubious software telling me my computer is slow (joke - it ranked 11th place world rank in one benchmark test), others have intrusive sound effects. All are unwanted.