I suggest you go to the “search” function the forum has here. Type in the question “is firefox compatible with Avast?” You will get numerous posts all talking about this same issue. Hope this helps you figure out what you want to do.
Not only Firefox, but Opera is also the greatest & safest alternative browser to IE, and of course, Opera is completely free.
I personally love and have been using Opera for years. In my opinion, Opera is far better than both Firefox and IE7 in many ways, i.e. Opera is amazing ergonomics surfing design, great tab surfing design (Opera is the first browser to use tab almost 10 years back), amazing small-fast-speedy performance and super-quick response both program itself and page rendering (also cache management), packed with some handy tools so you don’t have to install external extensions, etc. These make Opera the best browser for me and make Firefox is nothing but over-hyped browser period, IMHO. ;D
Of course, Opera isn’t perfect and has its own problems (i.e. some distorted page rendering especially with non-English pages or non-standard design pages), but it’s negligible for me, Opera fits my needs anyways.