Hi cinike, Firefox should keep the order of tabs within in each window, but the order of the windows on the Taskbar can change between sessions. It maybe is related to the order in which you last accessed them??

If the tabs within a window are in reversed order, that would be weird. Do you use any session management or other tab-managing add-ons that might not be working quite right?

This code is working for me in Firefox 65. You can adjust the two --tab-min-height definitions under the Height section to suit your needs (give them both the same value; default:33px; compact:29px; touch:43px).

Add code to the userChrome.css file below the default @namespace line.

   http://kb.mozillazine.org/userChrome.css
   https://www.userchrome.org/how-create-userchrome-css.html
   https://www.userchrome.org/what-is-userchrome-css.html 

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* only needed once */

/* TABS: on bottom */
#navigator-toolbox toolbar:not(#nav-bar):not(#toolbar-menubar) {-moz-box-ordinal-group:10}
#TabsToolbar {-moz-box-ordinal-group:1000!important}

#TabsToolbar {
 position: absolute !important;
 bottom: 0 !important;
 width: 100vw !important;
}

#tabbrowser-tabs {
  width: 100vw !important;
}
#main-window:not([chromehidden*="toolbar"]) #navigator-toolbox {padding-bottom: var(--tab-min-height) !important;}

/* TABS: height */
:root {
 --tab-toolbar-navbar-overlap: 0px !important;
 --tab-min-height: 33px !important; /* adjust to suit your needs */
}
:root #tabbrowser-tabs {
 --tab-min-height: 33px !important; /* needs to be the same as above under :root */
 --tab-min-width: 80px !important;
}

#TabsToolbar {
 height: var(--tab-min-height) !important;
 margin-bottom: 1px !important;
 box-shadow: ThreeDShadow 0 -1px inset, -moz-dialog 0 1px !important;
 background-color: var(--toolbar-bgcolor) !important;
}

#tabbrowser-tabs,
#tabbrowser-tabs > .tabbrowser-arrowscrollbox,
.tabbrowser-tabs[positionpinnedtabs] > .tabbrowser-tab[pinned] {
  min-height: var(--tab-min-height) !important;
  max-height: var(--tab-min-height) !important;
}

/* drag space */
.titlebar-spacer[type="pre-tabs"],
.titlebar-spacer[type="post-tabs"] {
  width: 40px;
}

/* Override vertical shifts when moving a tab */
#navigator-toolbox[movingtab] > #titlebar > #TabsToolbar {
  padding-bottom: unset !important;
}
#navigator-toolbox[movingtab] #tabbrowser-tabs {
  padding-bottom: unset !important;
  margin-bottom: unset !important;
}
#navigator-toolbox[movingtab] > #nav-bar {
  margin-top: unset !important;
}

/* hide windows-controls */
#TabsToolbar #window-controls {display:none!important;}

/* move caption buttons to right of Tab bar */
#main-window[tabsintitlebar]:not([inFullscreen="true"]) #toolbar-menubar[autohide="true"] ~ #TabsToolbar .titlebar-buttonbox-container {
 position: fixed !important;
 right: 0 !important;
 top: calc(6px + var(--tab-min-height)) !important;
 display: block !important;
 visibility: visible !important;
}
#toolbar-menubar[autohide="true"] ~ #TabsToolbar {padding-right: 100px !important;}

Note that there is a change in Firefox 71 that requires to add a "display: block;" rule to the #TabsToolbar container to get the tab on bottom.

• https://www.reddit.com/r/FirefoxCSS/comments/da72wv/psa_for_nightly_users_potential_breakage_or_even/

/* required for Firefox 71+ */
#TabsToolbar {
 position: absolute !important;
 bottom: 0 !important;
 width: 100vw !important;
 display: block !important;
}