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Currently the download literally is horrible in Firefox Android version. Firefox should introduce a proper in-built download manager or download manager extensions.
Another annoying thing about downloading is you cannot change the download location before the download starts. You cannot even rename the file you're about to download.
I think the Opera Browser has one of the best in-built download manager -
1. You can change download location,
2. You can rename file before downloading,
3. The downloads section shows seperate sections for separate file formats such as Image, APK, PDF etc.
4. You can copy links of the downloaded files
And many more.
Firefox should also add such a versatile Download Manager.