I've just tested it in a clean VM running Windows 7, and the MITM didn't work in current Firefox stable, but it did in IE. However, as far as I can tell, it only MITMs DV certificates, not EV. Also, when it MITMs a self-signed certificate, it generates an untrusted certificate, but it says it was generated by Avast, so the user could trust it more easily.

Also, in my experience, the free version of Avast considerably affects performance on slower machines (no SSD, earlier-gen CPU, etc.), but YMMV. It also tries to install Chrome as a default browser, a Google toolbar for IE, various "Secure browsing" extensions to other browsers and lot's of other annoying crap.