Why when I try to post something on my timeline it shows: “You can’t post this on your timeline.” What can I do?
It might be the case that Facebook has blocked or has certain filters working which ensures that certain objectionable content doesn't gets posted on Timeline.
Also,try posting after cleaning cache,cookies,history of Firefox.
maybe is a bug, have you tried on other browsers, on other computers. Also clean firefox from cookies, temp files...