Hi @MarjaE, the preference you referred "ui.caretBlinkTime", can be simple created, I guess isn't an issue not being there by default, is more like an enhancement thing. If you add this preference you should do that: after you open about:config, context-click (right-click) anywhere in the list of preferences and in the context menu > New > integer > give the name ui.caretBlinkTime > set the value at 0 (zero) or depends the scope of using, you can change this value.
I will add to this issue a component and change the type to Enhancement. Thanks for your contribution.
Test it if you can add this preference and you can see the effect of it then if some problem occurs will change it as an issue.