Hi Simon707
Taken directly from Tor Project: FAQ
In short, using any browser besides Tor Browser with Tor is a really bad idea.
I really don’t think there is such a thing as “secure enough”. Wouldn’t you would want to use a browser that was designed for security and anonymity. By using Firefox you are taking on more risk which is completely unnecessary.
Many users are under the false pretense that they can add plugins to Firefox, change a few settings in about:config, and they have themselves a Tor Browser; not even close! About the only thing they get is a false sense of security. Actually, it comes with something else: unexpected unwanted behavior. Like this user found out.
Obviously the choice is yours but you way want to reconsider your options. If you need more information you can read through the Crash Course in Security and Anonymity on the Internet ( AKA the Whonix Documentaion ).