Typically, when you click on a link to a post on Lemmy or Mastodon (or any other Fediverse application), it goes to the instance that was linked.
For example, I'm reading this on lemmy.world. If I were to link you to this post, it would load on Lemmy.world's site. Since you're reading the post on Lemmy.world instead of your account's instance (lemm.ee in this case), you can't interact with the post since you're not logged into Lemmy.world. This add-on would redirect you from Lemmy.world to the same post on lemm.ee.
(TL;DR) Adding this add-on would redirect you from Lemmy.world (remote instance) to the same post on Lemm.ee (your account's instance), which would allow you to interact with the post without having to find it on your account's instance manually.
It can now work on Firefox Mobile, but Firefox doesn't allow installing any extension for security reasons (idk if for compatibility reasons too), you can only install "recommended extensions". There's a way to install it on Firefox Nightly, but that's too advanced.
(Username and Password) or (jwt) are required for resolving /post and /comment. If Username and Password didn't work, go to your instance's cookies and copy your jwt.
I personally find it mildly amusing that picture 1 includes a post of Lemmyverse's largest Instance announcing it was hacked.
(I know it's true and it's a good thing that the announcement happened. But still...)
Anyways, with that out of the way. It could be an interesting browser flow, hope people like it!