Again, this is dumb. You aren't supposed to do this: Tor Browser is for Tor. Only do this if you want to get fucked in your arse.

First of all, install the Tor Browser via whatever. I'll be using tor-browser from my distribution's repositories.
Use whichever version you want, it should be pretty much the same (unless you're getting it from somewhere clearly sketchy).

Go over to about:profiles and create a new profile with a name like "I2P" and then open that profile on a new window.
In the new window, go over to about:config and press on "Accept the Risk and Continue". Write "socks" on the search bar.

Then, just edit these to these:

network.proxy.socks_port                = 4447 # socks proxy port provided by I2Pd.
network.proxy.socks_version             = 4
browser.fixup.domainsuffixwhitelist.i2p = true # So you don't search your links.
			

Simple, yeah? Well, the above is partially correct: You should be using the http proxy (127.0.0.1:4444) intead of the socks one.
Why? Because the socks proxy leaks your useragent, as well as the referrer. To fix this, use [bbergeron].i2p's guide instead.