How do I tell if a site is blocked by Firefox?

I work from a secure Microsoft Exchange email site. It was originally blocked by Firefox when I was hired about two months ago, but I was able to unblock it. Today I am unable even to get the login page to load. I'd like to find out if Firefox has it blocked again and how to permanently unblock it. It tells me it has "timed out" and that there may be a problem with the site (there is not), I should check my internet connection (it works for everything else) or see if it's blocked by Firefox.