@Flavius, I had this problem as well, and after contacting Discourse support I was able to resolve it for my own Firefox installation, so hopefully this will work for you too.
I am using an extension called ClearURLs, which removes common tracking parameters from URLs. Maybe you’re using it too?
If so, one particular setting in ClearURLs caused the problem, and switching it off has fixed the issue for me. The option is this one:
Make sure this is disabled, and hopefully you’ll be good to go. Let me know if not!
Daniel