Tabs flicker when text is overflowing
For some reason this happens when you move the mouse cursor around...
windows 10, FF100

I apologize for the bad crop 😃
Please specify your customization.
This happens to me too occasionally.
Now that I checked, screenshot used option userChrome.centered.tab.label.
There is a bug patch included in 6.0.0. https://github.com/black7375/Firefox-UI-Fix/commit/8ffcd5469786ab1e6806e190a9b1a732a8aba0fc
Try to use 6.0.0 above. I will open it again when a reproducible situation occurs.
Now that I checked, screenshot used option
userChrome.centered.tab.label.
It was happening me again today when using userChrome.centered.tab (not label) in 6.0.0.