I tried copy-pasting German text from Firefox three different ways, and none of them worked. The umlauts showed up as question marks. The text I used was https://www.volksliederarchiv.de/freut-euch-des-lebens/, which has plenty of umlauts. I copied from the visible text and pasted it into a message box. Then I pasted the same text into an "edit" bos, and then I pasted the source text into an edit box - on all three attempts, no cigar.

But if I use the ampersand code, ü - it will work:

ü

My favorite is the "s-zed ligature," ß - it will work, too:

ß

(done from Firefox)