SFOS 4.1 - XA2 Plus - Using Firefox from Aptoide - persistant screen flicker on top element of rendered webpages

REPRODUCIBILITY (% or how often): All the time
BUILD ID = OS VERSION (Settings > About product): 4.1
HARDWARE (XA2, Xperia 10…): XA2 Plus
UI LANGUAGE: EN(GB)
REGRESSION: (compared to previous public release: Yes, No, ?): Yes. No problem on 3.4 or 4.0.1

DESCRIPTION:

Bug = Firefox struggles to render webpages, creating constant screen flicker over the portion of the web frame that contains the top non-scrolling element of the page.
Fine in 4.0.1 - started happening with 4.1.
Not all webpages, but certainly youtube, twitter. and reddit.
Not really noticed it on the main page of reddit and youtube (just in-thread there), but it happens on main page and in-thread in twitter.

PRECONDITIONS:

STEPS TO REPRODUCE:

EXPECTED RESULT:

Render without flicker in top frame of rendered page

ACTUAL RESULT:

Flicker

ADDITIONAL INFORMATION:

(Please ALWAYS attach relevant data such as logs, screenshots, etc…)