It had been happening for a few days but I chalked it up to Firefox occasionally being weird. After a restart of the browser, and then another when an update was pushed, that my typing was still laggy on reddit meant it wasn't the browser. How to diagnose the cause?
Since I run various userscripts of my own crappy coding, that was my first thought: that I screwed one of them up and it was causing the trouble. Not in this case! Next I thought it might be RES, an ancient extension that, coupled with old.reddit.com, makes reddit usable. It has a live preview of rendered Markdown, so maybe its on-typing events were lagging. I really didn't expect it to be the case; much like I feel, RES is old and in maintenance mode. Turning that feature off unsurprisingly didn't change anything.
It was time to check out the browser's development tools ⸺ the performance tab in particular. This thing is extremely overwhelming in its detail, but it was easy enough to record myself typing a few things in a reddit comment box and then see spikes of CPU usage matching up to each keystroke. One of the graphs even had red bars helpfully labeled "jank" where the profiler was aware the browser was lagging like hell.
Clicking one of those "jank" bars opened up the enormous call stack below and pointed to the culprit: the isNodeFormFieldElement() function from the Bitwarden extension. While I have most options related to autofill disabled, I figured it was still the most likely to be lagging me. After all, on old.reddit.com each comment brings with it its own <form> element. With 200 comments loaded by default, that's 200 forms in the DOM plus several extra for other UI things. Perhaps Bitwarden was inadvertently running event listeners on all of those forms for every keypress.
I dug into Bitwarden's options and disabled autofill entirely for the reddit.com domain. All of the lag went away immediately. Problem noticed, found, and solved. Well, it's more of a "solve" in this case, but it's good enough for me. All that's left is to wait for the underlying issue to be fixed in the extension.