New Firefox Quantum has introduced new design and new UI bugs. While the bug with disappearing text is sporadic and font-scaling related, there is a PERSISTENT UI BUG, now traveling from version to version. Its name is: "Invisible TAB-bar buttons" They are simply white, with white background. This includes tab scroll arros, tab"+" button and recent list button, by default. It happens on EVERY WIndows7-based PC with EVERY Firefox installation in default configuration (no tweaks, no themes). I wonder, how the developers should hate Windows7, which is still installed on more than 30% of Win-PCs worldwide. P.S. double-posted this because the support server was not responding.