Hi SurfBlue714, welcome to the forum
Avast Online Security works perfectly well Firefox 42.0.
Try to reinstall Browser Protection :
1) Start - Control Panel - Add/Remove Programs - Avast - Change.
2) Change.
3) Uncheck Browser Protection
4) Continue.
5) Reboot.
Than follow the above steps again, but in step 3 you now check Browser Protection.
Let me know if this helps you.
Greetz, Red.