I think that is part of the "Photon" redesign (https://mozilla.invisionapp.com/share.../screens).

There might be some preferences for those elements, but I suspect there is a better place to learn about them than this site (most users here run release, esr, or maybe beta).

Based on a tip posted by cor-el in a different thread, putting this in a userChrome.css file in your current profile will switch the sidebar to the left side:

#sidebar-box {
  -moz-box-ordinal-group: 0; /* default value: 1 */
}

I checked it briefly in the "Portable" version of Firefox 55a1, but didn't explore around too much to make sure it is free of side effects.

See the daily thread in the mozillaZine Builds forum.

• http://forums.mozillazine.org/viewtopic.php?f=23&t=3029609&p=14744306#p14744306

/* Display the sidebar on the left */
#browser { -moz-box-direction: reverse !important; }

See also: Bug 1355331 Move sidebar to the right edge of the window

See also browser.photon.structure.enabled (currently false).

(edit: I found the same CSS code in my notes when I looked for it; see also )