Just installed Firefox 82 to check, looks fine to me. No empty bars, and it scrolls smoothly. A lot of the functionality seems to run on JavaScript, which is considered bad practice as it tends to cause slowdowns - that’s probably what’s causing the laggy behaviour in your case. Not so much a fault in Firefox as in the website.
I have been using Fennec-FDroid which is based on FF 82, and have exactly the problems you describe, with any site with some scrolling or similar (have been on the forum quite a bit on it). I think the Fenix update of FF wasn’t quite ready yet, so I reverted to old version of Fennec (68.12.0), with whichbI was perfectly happy before, and I’m waiting for update to 83, they say in the changelog it should solve some bugs and feature stability improvements.
Fennec F-Droid 82.1.1 on /e/ Pie (Android 9) …
Can’t confirm with deactivated Settings - Customise - Scroll to hide toolbar.
Can confirm with activated Scroll to hide toolbar … but I don’t like the toolbar vanishing and appearing again all the time anyway, so good thing Firefox 82 brought the setting back for this.
Looks like additionally to being an optical nuisance hiding the toolbar messes with JavaScript determining the correct window size to auto-fit the page into.
I’ve seen worse.
I’m with @robbert.f, the phone never was a performance monster, and JavaScript-heavy sites like the example here can be a bit sluggish.