Things possible in Firefox 57+ (by using CSS code in userChrome.css/userContent.css)

[Aris-t2]

As long as userChrome.css and userContent.css are supported by Firefox, Photon UI content can be modified using CSS tweaks, but CSS code can not create entirely new items, buttons or toolbars.

Most CSS code is copy/pasted from already available 'Classic Theme Restorer' and 'Classic Toolbar Buttons' add-on code.

Promo screenshots (Win7s AeroBlue colors are possible on other OSs too)

Tested / confirmed working in userChrome.css/userContent.css (at the moment)

• [Tabs] squared tabs (with rounded top corners)
• [Tabs] custom tab height
• [Tabs] (old) tab appearance/colors for active/non-active/hovered/pending/unread tabs
• [Tabs] remove ridiculous tab toolbar gap before first tab (in normal window mode without menubar)
• [Tabs] tabs not on top: tabs toolbar after navigation and bookmarks toolbars
• [Tabs] empty tab favicon: tab icon for tabs without favicons
• [Tabs] hide tabs toolbar for tab sidebar WEs (not visible on screenshots)
• [Buttons] navigation toolbar buttons: 'classic' button appearance (like offered by Classic Toolbar Buttons add-on)
• [Buttons] bookmarks toolbar buttons: 'default' button appearance
• [Buttons] default browser buttons: custom icon colors
• [Buttons] application/hamburger button: move button to the left
• [Buttons] application/hamburger button: cheat button into tabs toolbar
• [Buttons] application/hamburger button: custom label ("Firefox" on screenshots)
• [Buttons] application/hamburger button: different colors (Default, Aurora, Nightly, Transparent...)
• [Buttons] bookmarks menu button: label on bookmarks toolbar
• [Buttons] bookmarks menu popup: classic appearance
• [Buttons] custom images for toolbar buttons (like offered by Classic Toolbar Buttons add-on)
• [Buttons] downloads button: always visible (there is also an option for that in 'Customize...' mode)
• [Buttons] 'more' compact navigation toolbar buttons
• [Buttons] text / icon+text modes
• [Location Bar] location bar and search bar text size
• [Location Bar] location bars dropmarker at the end
• [Location Bar] change location bars icon padding
• [Location Bar] identity box colors for secure/valid connections
• [Location Bar] width/height tweaks to location bar popup
• [Location Bar] location bar popup: classic two lined results
• [General] hiding ui elements not removable by default (e.g. action menu in location bar, private mask)
• [General] context menu item labels instead of icons for back, forward, reload, bookmark this page
• [General] lw-theme support for squared tabs and 'tabs not on top' appearance
• [General] bookmarks toolbar: old padding
• [General] bookmarks toolbar: old height
• [Add-on Manager] alternative appearance
• [Add-on Manager] add-on version number in main lists
• [Preferences] alternative appearance
  ...
  Full list: List of features available in this project #2

Notes

• [Buttons] application/hamburger button 'in titlebar': space before tabs has to be adjusted manually (depends on custom button title length), 'Titlebar' and 'Drag Space' checkboxes have to be disabled 'Customize...' area)
• [Buttons] application/hamburger button jumps back to nav-bar, if menubar is visible (prevents overlapping glitches)
• [Location Bar] popup width has to be adjusted manually to match location bars width for normal, maximized and fullscreen window modes

Already in Firefox 57+ without installing WebExtensions (compared to Firefox 56 or older)

• [Buttons] star/bookmark button: button in location bar
• [Buttons] bookmarks menu button: disconnected from star button (see 'Customize...' area)
• [Buttons] back and forward buttons: disconnected from location bar
• [Buttons] back and forward buttons: separately movable within navigation toolbar bar
• [Buttons] stop and reload buttons: buttons combined and outside location bar
• [Buttons] sidebar button: one button for bookmarks, history and synced tabs
• [Buttons] application/hamburger button: popup 'closer' to the old one (no huge icons)
• [Buttons] downloads button: progress indicator below icon
• [General] flexible spaces (see 'Customize...' area)
• [General] drag space: space above tabs toolbar (see 'Customize...' area)
• [General] toolbar modes: compact, normal, large (see 'Customize...' area)

Things possible by installing WebExtensions

• [Buttons] Old 'Firefox' menu (WIP): Application Menu
• [Buttons] RSS button in location bar: Awesome RSS
• [Buttons] Reload/stop buttons in location bar: Reload in address bar or Australis-like refresh in URL bar
• [General] custom 'about:newtab' page: New Tab Override
• [Tabs] Sidebar tabs: Tabs Center Redux (additional CSS code in userChrome.css required to hide default tabs toolbar for now)

Not properly working using CSS tweaks

• [Tabs] custom tab widths: issues with overflow mode like on previous Fx versions, if modified through custom CSS areas

Not possible

• 'real' buttons in titlebar (moving buttons to a toolbar and cheating them into titlebar is possible)
• actually/manually moving existing buttons to different locations than toolbars (e.g. feed button in location bar, application/hamburger button to tabs toolbar)
• additional toolbars, add-ons bar, status bar
• custom scrollbars

[motfis]

Nice! I got my 'old' firefox layout back. Still need to configure some/many things though.
Btw, I have some questions:

1. if I want to change the firefox button (top left) color (in the CTR, we have option to change the color to blue/green/grey etc), which part of the css that I need to change?
2. if I use the application menu extension, is it possible to show the menu of application menu when I click the firefox button?
3. Small thing : the ff button seems a bit cropped in the chrome css version (https://imgur.com/a/F45zM)

Thanks for this chrome css!

edit: issue no.3 is fixed in v. 1.0.2

[Aris-t2]

1. userChrome.css > Application/hamburger button in Firefox titlebar
2. Popup swapping is not possible with CSS. Once application menu extensions popup will finished, I will add CSS code for it. At the moment most menuitems are grayed out.

[Arty2]

also possible with userChrome.css:

• Colorize URL bar in private tabs
• Move tabs to bottom of window

[Aris-t2]

Thanks.

The mentioned list was mostly about what of CTRs features are still possible in Fx57+, so it obviously does not include any exotic tweaks.

Colorize URL bar in private tabs
There is a tweak to colorize location bar in this collection (although its more general, not specifically for private mode only).

Move tabs to bottom of window
Moving "tabs" to bottom like this, is more like moving the whole main top content to windows bottom, resulting in a not-present "Firefox titlebar", partly visible panel popups and obviously all other toolbars being not on top anymore. Anyway, thanks for the code, the "-moz-box-direction: reverse;" rule looks very interesting.

[Aris-t2]

List of features
#2

Discussion
#3