Its a suggestion!
Some times when we are developing web on our local server (PC) and we check our web without having web tools opened we often miss console errors and thinking why things are not working properly so what about if we work in local environment and if anything goes wrong in console there should be a visual indication on Firefox.
Its a just suggestion which i think it would be a awesome little feature 
Thanks
Personally, I did not think this would be a great improvment. For my workflow, I just have the DevTools open (with Split Console mode) while developing.
But you might be interested in the Developer Toolbar (Tools > Developer > Developer Toolbar). It shows the number of errors - if there are any - in a red button (which you can click) at the right hand side. https://developer.mozilla.org/en-US/docs/Tools/GCLI Maybe not exactly what you need, but maybe it helps you.
Hello there,
The javascript-errors addon was just published and might be whatg you need: https://addons.mozilla.org/en-US/firefox/addon/javascript-errors/