[OBSOLETE stopped working with firefox 71]

The Tab Mix Plus addon stopped working with v57 :-(

1. Open about:config and set toolkit.legacyUserProfileCustomizations.stylesheets to true.
2. Open a new tab on about:support
3. Click the button next to “Profile Folder”
4. Go to the “chrome” directory at the profile folder, or create it if it doesn’t exist.
5. Create a file named “userChrome.css” (if it doesn’t exist) and open it in a text editor.
6. Copy the css code from github.com/andreicristianpetcu/UserChrome-Tweaks
   (copy of the css is at the bottom of this post)
7. Paste it into userChrome.css and save.
8. Restart Firefox

Update for v58: Changed .tabbrowser-tabs to #tabbrowser-tabs.

Update for v65: Removed #titlebar related stuff

userChrome.css

.tabbrowser-tab:not([pinned]) {
    flex-grow:1;
    min-width:150px;
}
.tabbrowser-tab,.tab-background {
    height:var(-tab-min-height);
}
.tab-stack {
    width: 100%;
}
#tabbrowser-tabs .scrollbox-innerbox {
    display: flex;
    flex-wrap: wrap;
}
#tabbrowser-tabs .arrowscrollbox-scrollbox {
    overflow: visible;
    display: block;
}
#tabbrowser-tabs .scrollbutton-up,#tabbrowser-tabs .scrollbutton-down,#alltabs-button,.tabbrowser-tab:not([fadein]){
    display: none;
}
#main-window[sizemode="maximized"] #TabsToolbar{
    margin-left:var(-tab-min-height);
}
/*
#titlebar,#titlebar-buttonbox{
    height:var(--tab-min-height) !important;
}
#titlebar{
    margin-bottom:calc(var(--tab-min-height)*-1) !important;
}
#main-window[sizemode="maximized"] #titlebar{
    margin-bottom:calc(6px + var(--tab-min-height)*-1) !important;
}
#titlebar:active{
    margin-bottom:0 !important;
}
#titlebar:active #titlebar-content{
    margin-bottom:var(--tab-min-height) !important;
}
*/

Based on instructions here.