The only way to modify ui is adding custom CSS code to userChrome.css and userContent.css files inside browsers profile folder.
Keep in mind CSS code can not create entirely new items, buttons or toolbars. It only can modify already present ui items.
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1. Find your profile folder ('profile names are different for everyone').
All OS
about:support > Profile Folder > Open Folderor
Windows
C:\Users\ USERNAME \AppData\Roaming\Mozilla\Firefox\Profiles\ PROFILENAME \ You have to make hidden files visible to see "AppData" folder or open "%APPDATA%\Mozilla\Firefox\Profiles\" from explorers location bar.
Linux
\.mozilla\firefox\ PROFILENAME \ You have to make hidden files visible to see ".mozilla" folder.
Mac OS X
~\Library\Mozilla\Firefox\Profiles\ PROFILENAME \ or~\Library\Application Support\Mozilla\Firefox\Profiles\ PROFILENAME \ 2. User styles belong into "chrome" subolder. Create it, if there is none yet.
\ PROFILENAME \chrome\ 3. Copy userChrome.css, userContent.css and subfolders into \chrome\ subfolder.
The userChrome.css and userContent.css files works like an options\configurations file. All main "features" can be enabled and disabled there.
Edit userChrome.css and userContent.css with any text editor (Notepad++ recommended on Windows) and enable btw. disable any feature you like by modifying, removing or outcommenting available "@import..." strings.
Restart browser after every change for changes to take effect.
Example
If you want "classic button appearance" for navigation toolbar buttons, the corresponding line has to look like this:
@import url(./css/buttons/classic_button_appearance_on_navbar.css);If you do not want "classic button appearance" for navigation toolbar buttons, the corresponding line has to look like this:
/* @import url(./css/buttons/classic_button_appearance_on_navbar.css); */Note
/* Code between this lines start and end symbols will be "ignored" btw. "outcommented" */1. Tools > WebDeveloper > Toggle Tools > Toolbox Options > Enable browser chrome and add-on debugging toolboxes
2. Tools > WebDeveloper > Toggle Tools > Toolbox Options > Enable remote debugging
3. Tools > WebDeveloper > Browser Toolbox
Open CSS files with a text editor. Look through the code and change/remove values the way you like.
Some files contain additional instructions to tweak the ui for individual cases.
Restart Firefox for changes to take effect.
Toolbar modes (suggestion: compact mode)
Customize mode > Density > Compact / Normal / Touch
Titlebar modes (suggestion: Firefox titlebar ['application/hamburger button in titlebar' only works in Firefox titlebar])
Customize mode > Title Bar > uncheck checkbox
Drag space above tabs toolbar (suggestion: disable drag space ['application/hamburger button in titlebar' works best without drag space])
Customize mode > Drag Space > uncheck checkbox
Bookmarks menu button on bookmarks toolbar
Customize mode > Toolbars > Bookmarks Toolbar
Customize mode > move 'bookmarks menu' button to bookmark toolbars end
Downloads button always visible
Customize mode > downloads button > click on button and uncheck 'Auto-hide'
Searchbar (suggestion: placed after location bar)
Customize mode > Search(bar) > move to navigation toolbar
Flexible spaces (suggestion: remove spaces after and before location bar)
Customize mode > grab and drag flexible space into palette
RSS icon in location bar
Install Awesome RSS WebExtension
(To revert changes right-click entry and select 'reset')
Tab audio icon
browser.tabs.showAudioPlayingIcon
Tab min width (suggestion: 100)
browser.tabs.tabMinWidth
Insert related tab after current tab (suggestion: enable / set to 'true')
browser.tabs.insertRelatedAfterCurrent
Hide 'http://' from url (suggestion: disable / set to 'false')
browser.urlbar.trimURLs
Open links in new tab/window
browser.link.open_newwindow.restriction > 0 (new tab instead window)
Preview tabs using 'Ctrl + Tab'
browser.ctrlTab.previews
Close window with last visible tab (suggestion: disable / set to 'false')
browser.tabs.closeWindowWithLastTab
Titlebar
browser.tabs.drawInTitlebar
HTML5 fullscreen warning
full-screen-api.warning.delay > 0 (reduces delay)
full-screen-api.warning.timeout > 0 (reduces delay)
Recently added bookmarks
browser.bookmarks.showRecentlyBookmarked
General animations
toolkit.cosmeticAnimations.enabled
Fullscreen animations for HTML5 content
full-screen-api.transition-duration.enter > 0 0 (reduces animation time)
full-screen-api.transition-duration.leave > 0 0 (reduces animation time)
Findbar: animated result highlighting
findbar.modalHighlight
Searchbar in 'about:preferences'
browser.preferences.search
Location Bar: search engines at popups bottom
browser.urlbar.oneOffSearches
Searchbar: open search results in new tab
browser.search.openintab
Reader mode
reader.parse-on-load.enabled
Geolocation (suggestion: disable / set to 'false')
geo.enabled
Pocket (suggestion: disable / set to 'false')
extensions.pocket.enabled
Container tabs
privacy.userContext.enabled
Flyweb
dom.flyweb.enabled
Font rendering
gfx.canvas.azure.backends > direct2d1.1,cairo,skia (old font rendering)
gfx.content.azure.backends > direct2d1.1,cairo,skia (old font rendering)
Anti fingerprinting
privacy.resistFingerprinting
Telemetry settings (suggestion: disable / set to 'false')
browser.ping-centre.telemetry
toolkit.telemetry.archive.enabled
toolkit.telemetry.bhrPing.enabled
toolkit.telemetry.enabled
toolkit.telemetry.firstShutdownPing.enabled
toolkit.telemetry.newProfilePing.enabled
toolkit.telemetry.reportingpolicy.firstRun
toolkit.telemetry.shutdownPingSender.enabled
toolkit.telemetry.unified
toolkit.telemetry.updatePing.enabled