I am creating userscript, webextension and etc. Current my script downloads several files with renaming. I know that there is some illegal character for windows file name like /?<>\:*|\". My script automatically replace this characters with full-width or remove. In android file system (like in linux) there is only one illegal character: / (used for folder address). But Firefox won't download file with name that contain characters like [],:?;*-/<|>". I use GM.download for userscript and for webextension browser.downloads.download with url and filename options. This also work like this in kiwy (android chrome with addons). Android Firefox can download file that contain this characters. This mean that you can download this file when link clicked, but can't download via userscript or addon.

Second problem is in desktop firefox if you download too many files (like 100 files) it will download 6 file at same time. But in android it download all file same time.