From what I have read, recent changes to YouTube doesn't support higher resolution on WinXP. If it still works on Chrome, maybe Google is "pushing" YouTube viewers to use their own browser?

But then again, Chrome is only supporting WinXP until the end of 2015. Enjoy it while while it lasts.

Might be the time to consider a newer operating system, like Win7 or 8.1 .

This is a re-post based on research done about 3 weeks ago. I have not double-checked that this still works because it's hard to get my hands on an XP device.

Until Firefox 39, YouTube served Windows XP users the Flash-based player. Starting in Firefox 40, YouTube now serves Windows XP users the HTML5-based player, which is very unfortunate because on Windows XP, Firefox can't decode MP4, so you get WebM 360p as your only format choice.

Assuming this is the problem -- you can right-click the player and look at the last line of the short context menu to confirm the player being used -- here are two possible workarounds:

(1) Use an add-on to force YouTube to serve you the Flash player

There are at least two extensions for this, you only need one:

• https://addons.mozilla.org/firefox/addon/youtube-flash-video-player/
• https://addons.mozilla.org/firefox/addon/youtube-flash-player/

(2) Use an add-on to tell YouTube you're still running Firefox 39

(A) Install the extension https://addons.mozilla.org/firefox/addon/ua-site-switch/ which admittedly has a confusing interface.

(B) Select and copy the old user agent identification for Firefox 39:

Mozilla/5.0 (Windows NT 5.1; rv:39.0) Gecko/20100101 Firefox/39.0

(C) While on a youtube.com page, click the little drop-down triangle for ua-site-switch and paste the old user agent string and then press Enter to save it

(D) Then reload YouTube bypassing cached files (Ctrl+Shift+r)

(E) Repeat B & C on https://www.youtube-nocookie.com/ to apply the hack to embedded videos