Users of previous versions of Firefox have found that if they had tabs below the address bar, the upgrade to version 65 broke that: tabs appear above the address bar.

userChrome.css code fix for normal browsing (See further down for code that works for both normal and Private browsing windows.)

For Firefox 65 users who prefer to have tabs appear below the address bar, the procedure is described in Forum Response - Tabs below the URL bar

Firefox's style or appearance can be modified by creating a chrome folder inside your Firefox profile folder and then placing a userChrome.css file inside, that includes a custom style rule. The rules you put in the userChrome.css file will override the default styles in Firefox.

with this caution:

Custom style rules might not work every time because of other factors (such as incompatibility with other custom style rules) beyond the Mozilla community's control. Your custom style rule might also stop working each time a new Firefox release comes out.

On my system, this is the path to userChrome.css:

/home/dkb/.mozilla/firefox/prefix.suffix/chrome/userChrome.css

Both chrome and userChrome.css are case-sensitive.

The following lines needs to be placed in userChrome.css:

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

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

Save the file and restart Firefox.

Notes:

• If you already have content in your userChrome.css, the first line is probably already present and doesn't need to be repeated.
• You need to ensure that either the menu bar or the title bar or both are visible. If both are missing, the tabs on bottom will overlap and obscure your bookmarks bar and the minimize/restore/close buttons won't be displayed.

userChrome.css code fix for Private Browsing

I found a slightly longer userChrome.css code in Reposition Tabs at bottom, directly above web page in Firefox 65.0?. It requires some modification by the user but it works for both normal and Private Browsing windows:

@namespace url("<a href="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul" rel="nofollow">http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul</a>"); 

/* 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: 29px !important; /* 27 --- adjust to suit your needs */
}
:root #tabbrowser-tabs {
--tab-min-height: 29px !important; /* 27 --- 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;
}

#tabbrowser-tabs,
#tabbrowser-tabs &gt; .tabbrowser-arrowscrollbox,
.tabbrowser-tabs[positionpinnedtabs] &gt; .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] &gt; #titlebar &gt; #TabsToolbar {
padding-bottom: unset !important;
}
#navigator-toolbox[movingtab] #tabbrowser-tabs {
padding-bottom: unset !important;
margin-bottom: unset !important;
}
#navigator-toolbox[movingtab] &gt; #nav-bar {
margin-top: unset !important;
} 

Note the px value of 29 in --tab-min-height: 29px !important works for me. You will need to adjust that for your system in both the lines in which that appears.

Other cautions as described above for normal browsing windows still need to be noted.

Firefox 65 with tabs below the address bar for normal and private windows: