Fixes/changes:

Change of the browser's GUID (Globally Unique Identifier).
The new GUID is {8de7fcbb-c55c-4fbe-bfc5-fc555c87dbc4}.
Allow extensions with both Pale Moon GUID and Firefox GUID to be installed natively (dual-ID system).
Pale Moon GUID blocks will have preference.
Disconnect of Pale Moon's "Firefox compatibility" version from Pale Moon's application version to maintain Firefox 24.* extension compatibility regardless of Pale Moon version.
Disable Firefox Compatibility mode by default.
This means Pale Moon will no longer have a Firefox/xx.xx indicator in its UserAgent string.
This may impact some websites that check browsers by UserAgent and possibly warn, complain or block you. You should contact the site's owners and request support for Pale Moon. Pale Moon will allow you to override the UserAgent on a per-site basis if you absolutely must visit the site and they absolutely won't cater to your freedom of browser choice.
Use the alternate sync server on a new system.
Current Pale Moon sync accounts cannot be ported over, so you will have to create a new account when updating to v25.
The previous server implementation will be retired on the very short term to free up infrastructure and reduce expenses.
Stop building Mozilla Corporation service tools and never verify MAR signatures (MARs are not digitally signed against internal certificates so it won't work anyway).
Stop building the WebApp runtime by default.
The use of "Web Applications" started from the command-line is such a niche feature that it has no business being in Pale Moon's main-line builds.
If you need the WebApp runtime for your specific organization and want to use Pale Moon, you can build Pale Moon from source with the feature enabled.
Stop supporting Windows XP.
By default, do not sync add-ons.
Syncing between different devices will likely not want you to sync the add-ons in use. There's a reason you're using different devices, after all.
Un-prefix CSS box-sizing
You can now use box-sizing:border-box box-sizing:padding-box and box-sizing:content-box to switch box-sizing mode on elements using CSS.
Implement image-orientation in CSS
You can now use image-orientation: {angle} [flip] in CSS to rotate images in 90 degree steps and optionally flip them.
Improve bookmark menu item-dragging.
Dragging bookmarks in the bookmarks menu is now more convenient (allow diagonal dragging, prevent tooltips from interfering, etc.)
(Fixes bugs 225434, 419911 and 555474)
Move the option to "use the classic downloads window" from status bar preferences to the main options window.
This way, it's easier for people to find and it's in a much more logical place. The classic downloads window will not go away any time soon in Pale Moon.
Use (i) icon for error console informational messages instead of (?).
Update branding images for official/unofficial logo, and some about: pages.
Add a new type of "blank new tab" page with logo-styling.
This logo page will be the default setting (instead of about:blank)
Add Opus audio to WebM
Add VP9 codec to WebM :!:


Bug/regression fixes:

Prevent error in removeobserver() for the padlock code when closing a window
Release XPCOM timer immediately after firing to prevent a race condition.
Android & any ARM processor: Always use integers for audio instead of floats.
Properly apply the use of high contrast themes on Windows 8/8.1
Prevent the accumulation of hidden about:blank windows in some situations.


Security fixes:

Properly derive/insert the host of a URL
Avoid negative audio ratios (can lead to exploitable crashes)
Avoid some root hazards in the style parser
Add is-object check to IonBuilder::makeCallHelper

http://www.palemoon.org/releasenotes.shtml