Open torrents from web browser in Linux

From AzureusWiki

How to load .torrents directly into Azureus from Mozilla / Firefox

In configuration, under Mime Types, create a new type:

MIME Type: application/x-bittorrent
Description: Bittorrent torrent
Extension: torrent
Open it with: /home/user/azureus/azureus (or wherever your azureus starting script is)

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Alternatively

This change can be made available system wide (to all users of all browsers), as follows:

Add the following entry to /etc/mailcap

  application/x-bittorrent; /usr/local/azureus/azureus "%s"

Change /usr/local/azureus/azureus to whatever your azureus startup script is.
Also, if your intention is that azureus should be available to multiple users on the system it should be installed somewhere where all users can access it, with appropriate permissions set.