Send to aria2 
This Firefox extension captures download links and sends them to aria2 server. You can download it here.
Usage
Right click the link that you want to download, and select "Send to aria2" option. If the link requires basic HTTP authentication, and you have the username/password saved in your credentials cache, they will be sent automatically with the link, otherwise you will be presented with the popup where you can enter your credentials. Warning: because your HTTP credentials are sent in plaintext across the network, it is strongly recommended that you run your aria2 server with TLS enabled, or put it behind TLS termination proxy (like nginx or HAProxy).
Client Configuration
For this extension to work, you have to specify at least JSON-RPC Path in the extensions preferences. If you are running your aria2 server on a domain www.example.org with default command line options, your JSON-RPC Path should look like this:
http://www.example.org:6800/jsonrpc
RPC Secret option is optional, but it's strongly recommended that you use it. To start the aria2 server that requires secret token for every job, pass it the --rpc-secret=some_secret_token command line option. Secret token is supported in versions 1.18.4 or newer.
Server Configuration
To start aria2 server, run the following command:
aria2c --enable-rpc --rpc-listen-all=true --rpc-allow-origin-all --rpc-secret=some_secret_token