For some extensions, yes. But the problem isn't so much sandbox from a security perspective, but the lack of access to Firefox internals. For VimFx the primary

For some extensions, yes. But the problem isn't so much sandbox from a security perspective, but the lack of access to Firefox internals. For VimFx the primary problem is that it makes major changes to how input is processed. It can't get the level of input access it needs from the WebExtension APIs. You can see this in the WebExtension versions of similar vim-style binding extensions, I've tried maybe a dozen and none of them work as you would expect. It is common for inputs to be missed by the extension depending on what part of the browser is focused at the moment.