The issue is not technical. It's just a chicken and egg problem. Most won't use bittorent unless it's stupidely easy to do. Remember that the average user don't know what an URL is and doesn't open new tabs willingly. Since they are the majority, they drive cost and benefits, so we must include them.
You couldn't, until Firefox 59. Before that, protocol handlers were not allowed to handle links to Dat/IPFS resources [0].
And while I agree with your comment regarding the chicken and egg problem, there are still some technical issues. As the shadowbanned sibling comment says, extensions don't have access to UDP/TCP sockets, meaning that you will need to run a gateway on your machine. See e.g. what dat-fox [1] does.
[0]: https://developer.mozilla.org/en-US/Add-ons/WebExtensions/ma...
You could, kind of, before Firefox 57 (or at least, at some point). Implementing nsIProtocolHandler/nsIChannel/etc. correctly was difficult (and probably not from JavaScript), and distribution problems meant nobody did it.
https://addons.mozilla.org/en-US/firefox/addon/overbiteff/
But, not possible anymore (without tricks).
It does not have access to TCP or UDP sockets.