Edit: I believe this is caused by #12909, not by scrolling before swiping between tabs
open 2 tabs
swipe left and right to switch tabs

not do any flicker
has a short flicker
Not a big deal, but yes, I have the same.
OnePlus 5 on stock ROM (OxygenOS 9.0.10 based on Android 9) + Magisk.
Firefox Nightly 200730.
Hold on I think this was a recent regression, unrelated to the scrolling
It was a regression. It worked fine in the first swipe implementation.
Seems to be fixed in the latest Nightly: 200801
ohhh.... you want QA to verify it. reopening.
It works fine for me.
Verified as fixed on Nightly 8/4 on two devices: Pixel 4 (Android 10), Htc Desire 820 (Android 6.0.1).
Most helpful comment
Not a big deal, but yes, I have the same.
OnePlus 5 on stock ROM (OxygenOS 9.0.10 based on Android 9) + Magisk.
Firefox Nightly 200730.