If you are in UK shouldn't you be signing in with GMX.co.uk anyway? I can't see any reason why you should have been blocked simply because you uninstalled/reinstalled Firefox but I could be wrong. I use Firefox 13 and GMX but log-in using co.uk - if I were to use GMX.com it defaults to co.uk anyway. You may find that things have returned to how they were - but there is only one way to find out!
Dell XPS 13 9370 (2018) review