I had high hopes for this but in its current state, I rarely use it (and just now disabled it). The limit is absurd (hardly any width).

Firefox needs a split screen mode like Vivaldi, where tabs can be split into 2/3/4 screens. Let's start with 2 for now but allow us to remain in our session and not log us out. Let the page not reload just for opening it in a split view. This actually signs you out of the website (the one in the side view). It reloads the tab when you add it to the side view. It's all over the place when it comes to websites where you're signed in.
It's really a very bad implementation and it's clear that care is not given to this add-on, which is unfortunate.

Allow us to split screen an .html file (that I open from my Desktop, which opens in Firefox) with a on-the-Internet tab.

I really want a real split screen add-on, one that also would let us resize to our heart's content and not be limited as we are now. One that displays the tab without reloading, without signing us out, without a new cookie or whatnot. Look at Vivaldi's Page Tiling and imitate that, that's all!

Edit: Use Tile Tab add-on, it's good enough for now but not like Vivaldi's and Edge's implementation, which are the best.

Edit 2 Sept 2022: This broke Asana and it would get stuck at the Download Asana App rather than log me in to the task view.

Edit: July 2024: I tried it again for 2 months but is still breaks websites and also Temu remains in mobile mode. I couldn't figure it out until I recalled that this is active. Deactivated right away. What a buggy mess! Made by Mozilla themselves too lol. Embarrassing.