Good question! It's a bit confusing because these two browsers, Firefox and Reddit, use a similar spellcheck feature.
So let's break it down:
Firefox uses a spellchecker called Hunspell. It is built-in to Firefox and will automatically check the spelling of words as you type them in the browser. You can customize it to fit your needs, and it will check your spelling across all pages.
Reddit also has a spellchecker called Reddit Spelling Checker. It is a 3rd-party add-on to be installed before you can use it. Also, it only checks spelling in the comments section, so you have to be careful to double check your spelling on posts.
So, to answer your question: it depends which feature you are utilizing. If you are typing in the comments, it could be Reddit's spellchecker. If you are typing in other areas of the page, it could be Firefox's Hunspell.
I hope this clarifies things for you!