L.S.
Not to come to the defense of avast's, but certainly particular software code can be more FP-prone than others.
When code has been signed properly it is also much easier to avoid such mishaps.
Javascript can be a can of worms with suspicious and malicious activity bordering closely.
The final verdict depends on quite some factors and circumstances.
Time pressure in releasing definitions also counts.
Every av-vendor has issues, and whenever they tell you they haven't,
they have placed themselves outside normal life's reality.
Never take things for granted, because you cannot. That's real life for ye.
Check and counter-check. An that's why we have these here forums.
polonus (volunteer 3rd party cold recon website security-analyst and website error-hunter)
P.S. On an earlier MBAM-PUP-detection:
https://forums.malwarebytes.com/topic/251908-detecting-prefsjs-on-my-firefox-profile/On the use of prefsCleaner:
https://github.com/arkenfox/user.js/wiki/3.5-prefsCleaner