I'm currently running KDE Plasma 5.13.5 on KDE Neon and on Fedora 28. Both exhibit the following:

When I run a KDE Plasma desktop, I cannot adjust the font that is used for my browser tabs and menus. Note: this has nothing to do with adjusting the fonts used for browser content or with adjusting system fonts -- those are easily adjusted and are not an issue here. The default font for browser tabs and menus is small and difficult for me to use -- old eyes, I suppose. I think there are other users who experience this problem. Also, the default font for browser tabs and menus is unattractive -- it mars the beauty of the KDE environment, in my opinion.

This problem seems to be KDE-specific. It never occurs when using Gnome or Xfce, regardless of the operating system (fedora, arch linux, debian-based) and regardless of the browser (chome/chromium, firefox). In all of those cases, the browser tab/menu font is the browser content font, which is what I want. On the other hand, this problem always occurs when using KDE, regardless of the operating system and regardless of the browser.

Am I missing some obvious setting? Any ideas about what causes this aberration and whether it can be fixed?

You should be able to configure the browser UI fonts via System Settings -> Gtk.