Firefox Custom Theming

https://www.userchrome.org/how-create-userchrome-css.html

Open your currently active profile folder (goto about:support page and check profile directory) or get profile folder from about:profiles and create an new profile and make that default.
Create a new folder named chrome
Create a desktop shortcut (alias) to the chrome folder for easier future access (optional)
Make sure your OS is set to show you file extensions like .txt and .css
Create a new text file inside the chrome folder named userChrome.css
Change a preference in Firefox so it looks for your files at startup (in Firefox 69+)
1. In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.
2. In the search box above the list, type or paste userprof and pause while the list is filtered. If you do not see anything on the list, please ignore the rest of these instructions. You can close this tab now.
3. Double-click the toolkit.legacyUserProfileCustomizations.stylesheets preference to switch the value from false to true

Replicate Chrome behaviour for clipped tabs:

[about:config] Set browser.tabs.tabClipWidth to 83 (default is 140).

Replicate Chrome's "Not Secure" text on HTTP:

[about:config] Set security.insecure_connection_text.enabled to true.

Custom Theme CSS Files: https://github.com/Etesam913/rounded-fox https://github.com/Khalylexe/Firefox-Rounded-Theme https://github.com/datguypiko/Firefox-Mod-Blur **BEST

Security and other mods: https://github.com/yokoffing/Betterfox

List	Description
Fastfox	Increase Firefox's browsing speed. Give Chrome a run for its money!
Securefox	Protect user data without causing site breakage.
Peskyfox	Provide a clean, distraction-free browsing experience.
Smoothfox	Get Edge-like smooth scrolling on your favorite browser — or choose something more your style.
user.js	All the essentials. None of the breakage. This is your user.js.

other settings in about:config

Setting	Value	Description
mousewheel.default.delta_multiplier_y	90	Adjusts scroll increments
network.http.max-connections	1500	Increases internet speed
network.http.max-persistent-connections-per-server	10	Increases internet speed
browser.urlbar.showSearchSuggestionsFirst	false	Puts Firefox suggestion on top of URL drop menu
browser.tabs.loadBookmarksInTabs	true	Loads bookmarks in new tab when clicked on
browser.urlbar.suggest.calculator	true	Adds calculator functionality to URL box
accessibility.force_disabled	1	Addresses memory leak, but disables accessibility hardware
full-screen-api.transition-duration.leave	0 0	Removes the transition when leaving fullscreen
full-screen-api.transition-duration.enter	0 0	Removes the transition when entering fullscreen
full-screen-api.warning.delay	-1	Removes fullscreen warning if cursor is at edge of screen
media.hardware-video-decoding.force-enabled	true	Enables hardware acceleration for fullscreen media
middlemouse.paste	true	Middle clicking field pastes clipboard content
full-screen-api.warning.timeout	0	Removes the fullscreen notification
security.dialog_enable_delay	0	Removes delay before clicking on downloaded items
nglayout.initialpaint.delay	23	Waits before webpage loads, reduces flash bang effect
findbar.modalHighlight	true	Dims screen when searching site (Ctrl+F)
dom.webnotifications.enabled	false	Removes website notifications prompts
browser.display.os-zoom-behavior	1	Affects zoom controls (Ctrl + mouse wheel)
network.http.max-connections	1800 - 20000	improves download speeds
network.http.max-persistent-connections-per-server	10-12	improves download speeds