Firefox has a ton of different downloaders and some of them are multi app compatible, I know I've seen extensions and add-ons for both opera and chrome in Firefox's add-ons and extension sections, not all of them work properly though so just a heads up and I really don't know if these packages have been maintained, however it's a place to start from at the very least.
Please let me know how your search goes.
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RE: Desperately searching for Downloadhelper ExtensionOpera add-ons