The default system theme on Ubuntu 21.04 and 20.10 has very low contrast for the tab strip icons, specifically the new tab, overflow scroll arrows and tab manager icons.
Regressed by Bug 1705849.
Let's back out bug 1705849 rather than making our Linux users suffer with this all weekend. I'll ask the sheriffs to do that.
I have managed to reproduce the issue on an affected Nightly 90.0a1 (2021-04-24).
Confirming this issue as verified fixed on Ubuntu 20.04 using Firefox 90.0b2 (2021-06-01).