I've noticed that this doesn't happen on Microsoft Edge, so I guess this is a Firefox only problem. Seeing that this hasn't been blogged about before, I suppose this could be a problem on my side.
Parts of the website are completely unreadable, unless text is selected. How can I fix this?
Cannot reproduce with Firefox on my machine (regard of whether I logged in or not). Could it be that you enabled some user script or user style sheet?