Comment # 2 on bug 1160331 from Maximilian Trummer

https://build.opensuse.org/package/show/openSUSE%3AFactory/MozillaFirefox

I've tested it with a Ubuntu 19.10 live DVD now, and both Firefox 71.0 (snap
package) and 69.0.3 (native program) work fine with '#' in file names.

Therefore I suspect the culprit is openSUSE's KDE file dialog patch for
Firefox.