I’m using F38 KDE, and since the release date, the control buttons on firefox are blurry. I tried switching to X11 that was slight less blurry but issue remains same. I reinstalled from scratch as well but no help.
BUT!! Gnome has no blurry issue at all. I tested it myself.
What is your display scaling (System Settings > Display and Monitor > Display Configuration: Scale)?
Mine is usually 125-150%.
(GTK apps do not always behave nice when scaling is other than 100%; didn’t notice this with Firefox, as the buttons are so small, but with Libreoffice).