5 years later and Avast still hasn't fixed this, or bothered to respond. Might be time for a refund
As for the OPs comment:
" I have a feeling it might have something to do with the Avast password thingy."
Not much clarity there as far as I know the " I have a feeling it might have something to do with the Avast password thingy." is avast program settings only, nothing to do with browsing, so without clarity how can anyone answer.
Ever thought posting in a 5 year old topics is pretty pointless, much has changed in that time.
Not only in Firefox and Avast, but there is no clarity as to what the cause is. I have been using Avast Free for over 17 years and Firefox pretty much from the early days and I have never experienced this issue. Not to mention the OP hasn't logged back in from when they posted.
So if you have an issue I suggest you create your own new topic and give details there.