(Mostly written to prevent me forgetting)
In about:config add this setting ui.caretBlinkTime of type integer. Then set the number to how many milliseconds between blinks.
250 is very quick, 500 is about normal. I tend to go for about 400. Set it to 0 if you don't want it to blink at all. Click the ✅ button and your caret blink speed will immediately change.
You can see more configuration options in the source code.
