I am not a big fan and user of the Mozilla Firefox though I used to be years ago. I am more into Chrome and now also into Brave which is just using the Chrome open-source platform. This is quite alarming and good thing that the problem is now fully traced and hopefully already fixed as I am sure that many people into cryptocurrency are also using Firefox. This is just one of the many risks that those in the cryptocurrency industry are facing every day.
For me, Firefox is safer to use than Chrome which is monitored by Google all of your activities online are being tracked according to this article "
Everything you need to know about Google's My Activity pageSo it is not safe to use as your browser and don't use it to install your wallet through plugins. To the fact that it's not advisable to use a wallet as a plugin in any browser because it's not safe. Compared to use a desktop wallet or any official wallet of every crypto.
There are many users who uses Brave browser just because they can earn BAT token from advertisement since advertisement has tracker it means they are still monitoring your browsing activity. So for me, they are still not safe to use.