For I2P specifically, there's also the Firefox extension, I2P in private. When it sees an I2P address, it opens up a configured container. (Or however that works in Firefox. I don't think container is the right word.)
If your running linux do this:
1. install tor and i2pd "sudo apt install tor" and "sudo apt install i2pd"
2. Find the proxy settings in FireFox (settings -> general -> network) or at your system level (settings -> network -> network proxy -> automatic)
3. Paste this URL as the configuration URL https://cdn.hzrd149.com/20ae592ff0761399537dbc20089003e3ed52f16494ebfe1ae759f5527d2c53ba.pac
4. Access any .onion or .i2p site (or nostr relay) using FireFox or any other browser on your system
FireFox network settings:
GNOME network settings
