Older Firefox Browser - Not Accepted Now
I'm unable to login because Tumblr now requires newer versions of browser software - both Firefox & Safari when running Mac OSX. So if you're like me with Mac OSX 10.6.8 and Firefox browser version 48.0.2 - it won't work at all with that old version of Firefox now.
With Safari, version 5.1.10, Safari seems to work at first - but Tumblr will NOT behave as expected, which is pointed out by warning message from Tumblr at top of Tumblr web page. Again, this was on Mac OSX 10.6.8
If you are like me, one of your next trips will be to buy a new computer - with latest OSX version, and capable of holding latest (or near latest) versions of both Firefox and Safari.
Bottom Line: Tumble has updated their system in a way that no longer accepts old versions of Firefox and Safari, as described above.