Installed, then visited https://de.wikipedia.org. Got forwarded to: https://de.m.wikipedia.org. Nice!
Nothing found to switch wmlbrowser off, so deactivated it.
Still be forwarded to m.wikipedia.org after deactivation. So removed wmlbrowser again. Rubbish!
Still be forwarded to m.wikipedia.org after removal. Hellfire!
Seems to me that wmlbrowser appends ",text/vnd.wap.wml;q=0.6" to value of "network.http.accept.default". After restoring the value to default I could see https://de.wikipedia.org again.
Maybe the strange behaviour is depending on the wikipedia site, which may ignore the "q"-parameter.