You say you have the latest version but you don’t say that is regular, beta, alpha or nightly, etc.
I have FF 40.0.2, normally I don’t run the browser add-on, but I have just dons a change and included the Avast Online Security (AOS) add-on. FF had a bit of a whinge when installing an add-on not from the mozilla source, but I was able to accept it.
On rebooting FF looking at the add-ons the AOS add-on isn’t disabled, it is however flagged as unsigned, but it is running and displaying the AOS icon at the top of the pages I visit.
Well it certainly has for me even with FF43.0.x (where the signing od add-ons is enforced) as the firefox Avast On-line Security (AOS) add-on has been signed.
What avast version are you using ?
What is your firefox version number ?
Is it the 32bit or 64bit version of firefox ?