It's fine. Actually, this is a bug report.

Vim-Vixen is not disabled entirely after Shift-Esc, needing, instead, for the Vim-Vixen to be disabled by opening about:addons and disabling vim-vixen there.

One example. Open a page in the online latex site "overleaf.com". If I want to use the vim interpreter built into overleaf.com, first clicking 'shift-esc' to disable Vim-vixen is not enough. Some keys, like Ctrl-[, are STILL modified, so overleaf.com does NOT work without vim-vixen disabled via about:addons.

If a plugin has a key stroke to toggle its on/off state, then, when the addon claims to be 'off', inactive, it should REALLY be 'off', inactive.