I have used Avast Free for more than five years, but, Avast seems to have compatibility problem with Firefox and MPC HC recently, especially with Firefox.
Everytime I installed Avast Free, Firefox would crash directly, even if I have updated my firefox from Version 51 to Version 54. Don't kown if this was individual problem of my computer, or is already an universal problem.
Compatibility problem with MPC HC could be solved with MPC update from 1.7.10 to 1.7.11
Plattform Windows 7 with SP1
Problem is solved: with an update of IE from IE8 to IE11. What can I say about this?
Crash report (in German) :