libx11 1.8.2 causes firefox to freeze when highlighting text

My platform is Arch Linux, running KDE, X11, Nvidia GPU.

I am getting intermittent freezes in firefox. It appears to happen when I highlight text. When it does happen, firefox becomes completely unresponsive. I haven't observed it happening in other applications, but I also don't highlight nearly as often in other applications.

I've narrowed down the package that causes the issue to Arch Linux's upgrade from libx11 1.8.1 to 1.8.2.

Initially a bug was opened in firefox, documented here: https://bugzilla.mozilla.org/show_bug.cgi?id=1800903