How are you setting font size? I’m running Plasma 6.3.5 and 150% desktop scale looks identical to 100% with a 150% page zoom in Firefox. Just scaling fonts sounds like it would make everything non-text appear the wrong size.
I do notice a weird behaviour where Firefox renders at 200% scale when it’s entirely within a display at 100% scale. If it overlaps another display at all then it uses the appropriate scale for each monitor. Maybe that’s a setting somewhere.
How are you setting font size? I’m running Plasma 6.3.5 and 150% desktop scale looks identical to 100% with a 150% page zoom in Firefox. Just scaling fonts sounds like it would make everything non-text appear the wrong size.
I do notice a weird behaviour where Firefox renders at 200% scale when it’s entirely within a display at 100% scale. If it overlaps another display at all then it uses the appropriate scale for each monitor. Maybe that’s a setting somewhere.