Add comment about Firefox CPU usage in FAQ

README.md

@@ -284,6 +284,9 @@ It is common to use `egui` from a game engine (using e.g. [`bevy_egui`](https://
 but you can also use `egui` stand-alone using `eframe`. `eframe` has integration for web and native, and handles input and rendering.
 The _frame_ in `eframe` stands both for the frame in which your egui app resides and also for "framework" (`frame` is a framework, `egui` is a library).
 
+### Why is `egui_web` using so much CPU in Firefox?
+On Linux and Mac, Firefox will copy the WebGL render target from GPU, to CPU and then back again: https://bugzilla.mozilla.org/show_bug.cgi?id=1010527#c0
+
 
 ## Other