Try harder to get the desired suggested filename in the Firefox add-on's download dialog

If the `Content-Disposition` header is missing, Firefox's attempt to suggest a filename does not always get it right.

For instance, the following URL:
`http://example.com/download.aspx?link=path/to/nice-filename.pdf&whatever`
results in the following suggested file name in the download dialog:
`download.aspx`

With this commit, the suggested file name becomes:
`nice-filename.pdf`

Fixes #3161