How Firefox Quantum looks like in Windows XP.

I installed the browser (Mypal) and installed the Photonic theme along with Toolbarize Menu Button and a style for it.

Firefox Quantum Look for Mypal (WinXP)

UPDATE 2020-Feb-18: Pale Moon now supports the new YouTube interface since they have announced killing the old version...

Modern Chrome Interface for Windows XP?

If you are on Windows XP and Chrome 49, You have to enable these flags to have the Material Design feel!

Flags listed here:
Material design in the browser's top chrome - Material
Extension toolbar redesign - Enabled
Enable Material Design downloads - Enabled
Enable Material Design extensions - Enabled

There is the result of Chrome 49 with Material features enabled (Windows XP).

Chrome 49 with Material Design UI enabled

Chrome 49 with Material Design UI and any theme

Also, you can override the user agent string to the current version of Chrome (link here: User-Agent Switcher and Manager).

If you are on YouTube, The layout site goes blank. So there is a way to fix this.
Tampermonkey: Install from Chrome Web Store
YouTube - Restore Classic: Install from Script here

Other than that, Some other Chromium-based browsers were based on v49 and v50, I tested Slimjet 10 on XP...

Slimjet 10 with Material Design UI Flag enabled

And also, It works on any Windows versions such as XP, Vista, 7, 8.x, 10, and Linux of course...

List of Chrome Flags to Improve the Browsing Speed (Versions 34-46)

There are some flags listed in older versions of Chrome (34-46).

• Fast tab/window close - Enabled
• Experimental QUIC protocol - Enabled
• Enable Pinch scale - Disabled
• Hardware-accelerated video decode - Disabled
• Stale-while-revalidate cache directive - Enabled
• Maximum tiles for interest area - 512 (any RAM)
• Offline Cache Mode - Enabled
• Simple Cache for HTTP - Enabled
• Number of raster threads - 2 or 4 (cores)
• Disables the new video rendering path for video elements - Enabled

How to Get the Upload Button Back on YouTube (Firefox)

If you don't like the new "Create" button and you should want the "Upload" button back? There is a way to do this!

New String: general.useragent.override.youtube.com
Modified String: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:52.9.1) Gecko/20100101 Firefox/52.9.1

Screenshot of latest Firefox with user-agented Firefox 52.9.1.