I evaluated a few different options including Quill, but this problem never would have really come up in my prototypes. The performance issues really are only apparent when you have an obnoxious number of TinyMCE editors on a single page.

The reason TinyMCE won out was because they made self-hosting easy under license terms that my employer was OK with, and wrapping the whole thing in a Vue component only took me a few hours.

Do you need to have them all active at once? One option could potentially be to enable instances on click and disable on blur or similar.

FWIW, TinyMCE is a bit long in the tooth these days. I implemented my own from-scratch WYSIWYG editor back in 2010, and it wasn’t even the best option then (ckEditor was much better), and many people were using the unreliable lightweight jquery based editors because TinyMCE was considered too heavyweight.

I wouldn’t like to say for sure that’s it’s the source of your problems (Firefox’s contenteditable implementation is also “quirky”), but it could well be.