I am also using the asuswrt-merlin software and I would more attribute the change to the regular re-writing of the software.
(Asus & Merlin are 'fixing' the firmware on a regular basis from my reading of the changes on smallnetbuilder.com. I have upgraded 3 times already)
Additionally, I am using the latest version of Firefox as I upgrade as soon as the newest version is notified.
I did think about the caching idea, as per kitz, but thought surely something so basic could not be the case as it would impact lots of sites/webpages and would have been flagged up by now.
Maybe I am wrong.
Kitz I appreciate your stance but Avast is much better and I know they did have a problem with being over-vigorous with their classifications as did many AV companies, Mcafee included.
If this problem is NOT Avast but Firefox I will change back.