Cannot log into Tumbler via Firefox.

I've just started using Firefox about a month and a half ago. I like it, except, ever since my change from Internet Explorer, I cannot log into my Tumblr account. I use the correct email and password, but it continually redirects me to the same log-in page.

I know this is not due to user error, because it works just fine when I try logging in via IE. Unfortunately, I don't want to have to use IE just to visit Tumblr.

Any help?