Hi,
For what it's worth, I'm experiencing the exact same issue, and with exactly the same specifics as posted by: jrethorst.
I'm also on a Macbook, but late 2007, and still running 10.6.8, and my Avast version is 12.9.
This issue started for me a few weeks ago as well.
All of a sudden I started getting loads of what must be false alerts on random, and what must be safe websites, many that I've visited before, and the site types run the gamut, even including government websites.
I can't think of anything that I might have changed on my end that would cause the problem.
It seems the age of my computer and the age of my OS are certainly causing more problems than ever as they age out and are no longer supported or sent security updates.
This problem also ends up effecting my search engines since I can't update them any more because of the age of my OS.
I don't know why, but it seems as though Avast is acting sort of like an ad-blocker, or at least the infection alerts in some sense may be triggered by something as "benign" as an ad, no matter how intrusive and annoying they personally may be.
It's really a nuisance having to have to click on all these false alerts on so many websites to remove them, although I haven't disabled Avast yet as I assume/hope it is still working properly to detect any actual infections, malware, etc..