Font rendering in Canvas2D is slow plus you have to reinvent the wheel with things like word-wrap/new line. Using a texture based font in Canvas/WebGL is a lot faster and is used in many (older) games, but with the obvious downsides.
It's fast enough, and newline/wordwrap are not that hard to implement.
Do you really expect them to fix those issues? I'm not holding my breath. In the mean time, I've got a text editor that works identically in almost every browser, with very little browser-specific code, in VR.
Mozilla could fix the issues without hurting their business. Their legacy Thunderbird email client as well as various FirefoxOS system apps would profit too.
Microsoft, Apple and Google all have Office cloud apps and a really bug free contentEditable implementation would be a competition to their online Word/Pages/Docs. Google Docs v1 used contentEditable, but with v2 they coded a page layout render JS library.
I'm just saying, these issues have been issues for a long time, and to your point Mozilla does dog-food its own contentEditable (especially RE: the Ace editor) and it still hasn't seen fit to fix these issues. Yeah, it's clear that it's in their interest, but I think they've all demonstrated that it's just not a big enough deal to them.
Browsers should finally fix all issues of contentEditable API: http://www.quirksmode.org/dom/execCommand/ , https://developer.mozilla.org/en-US/docs/Web/API/HTMLElement... , http://caniuse.com/#feat=contenteditable