Consider dropping CSS hack for uBlock Origin sidebar

This:

ublock-origin (1.17.0+dfsg-1) unstable; urgency=medium

  * Add do-not-open-sidebar-on-first-start.patch and prevent that the sidebar
    in Firefox opens on first startup. This feature only works in
    Firefox >= 62. (Closes: #909493)

… should allow us to revert 3fc4fb4757222e5bf494eb41801862ba8d0b80c5 and e945d0c9dd331bf63edc576ace154df4aaff19a1, which should in turn fix #15924 (closed) (our Tor Browser exposes a different screen size than a pristine one).

Feature Branch: bugfix/16206-drop-ublock-origin-css-hack

Related issues

Related to #15820 (closed)
Related to #16973 (closed)
Blocks #16209

Original created by @intrigeri on 16206 (Redmine)

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