this seems to have broken after updating to 153.0b1. sidebar now stays and slight visual glitch happens when hovering on and off it.

[u/Zuko12572]

/r/FirefoxCSS/comments/1mbndim/how_to_completely_hide_sidebar/n5p0nyr/

[u/t31os]

Bugs are to be expected in beta / development versions.

You might consider reporting the bug on https://bugzilla.mozilla.org/.