Let's see here......
Firefox is a web browser, not an email program.
So how can you change any settings in Firefox that will have any affect on email?
If you are looking at email in a browser, then you either have what is called "internet email" (e.g., yahoo, hotmail, gmail, etc.) or you are logging in to your Internet Service Provider account on the web instead of using your email client application on your pc. If you have "internet email" then there is no way they are going to let you log on to their server. If you have ISP email available online then it's possible your ISP can tell you if the emails are legit or not with a phone call to tech support.
Either way, Firefox settings shouldn't have anything to do with it. Of course, there is obviously a lot I dont know about Firefox....... so please fill in my gaps if necessary with real knowledge.
I only commented because no matter what I do or who I email or who I talk to on the phone, Line 6 will not remove me from there email list. I get there crap "offers" every month like clockwork. Last time I complained about it was a couple weeks ago. Seems like first of every month. If they cant figure out how to do a simple remove from the spam list, they need to hire a new programmer. I got rid of everything I had that says "Line 6" on it, and now I cant get rid of Line 6.....