It’s true, there is support for inline spell check in Firefox!
When I upgraded Firefox yesterday my SpellBound didn’t work so I did a little searching and found the developers preview of SpellBound. Not only did it work in Firefox 126.96.36.199 but it came with a new feature; inline spell check!
Now, just like Thunderbird, when you type in a text box, misspelt words underline in red. The only downside is that you can’t right click and get suggestions but I’m sure they are working on it. Either way, this is fantastic!