NS_S_CHANGENOTICE (0x000D2F0D): Fix Firefox Internal Error

Quick answer

For advanced users: This is a benign internal status code, not a crash. Disable all extensions, restart Firefox in Safe Mode, and if it persists, create a fresh profile.

What's actually happening here

You're not looking at a typical Windows error. NS_S_CHANGENOTICE (0x000D2F0D) comes from Mozilla's own networking code. It's defined in nsError.h as a notice that the underlying document changed during a load. Think of it as Firefox telling itself, "Hey, that page you were fetching just got updated, so discard what you had."

Because it's internal, you almost never see it in the browser window. It shows up in three places:

The browser console (Ctrl+Shift+J)
Error logs from extensions that are poorly handling async loads
Occasionally as a pop-up from a misbehaving WebExtension (usually an ad blocker or a download manager) that's intercepting requests it shouldn't.

The reason you're seeing it now is probably a combination of an outdated extension and a stale profile cache. Firefox changed how it handles document replacement in version 89 (June 2021), and some older extensions still expect the old behavior. When they call webRequest.onBeforeRequest and return a redirect, the internal logic fires this notice.

Fix steps (in order of likelihood)

Restart in Safe Mode. Click the hamburger menu → Help → Restart with Add-ons Disabled. If the error disappears, you've got a culprit extension. Now disable half your extensions, restart normally, and test. Binary search works fastest—disable half, check, repeat. When you find the bad one, check for an update first, then remove it.
Clear the cache and refresh the profile. Even if extensions aren't the issue, a corrupt cache entry can trigger this. Go to about:profiles in the address bar. Click "Launch profile in new browser" to test without losing your current setup. If that's clean, go back to your main profile and clear cache via Settings → Privacy & Security → Cookies and Site Data → Clear Data (check only Cache). This wipes the cached document states that might be throwing the notice.
Delete the startupCache folder. Close Firefox completely. Press Win+R, type %LOCALAPPDATA%\Mozilla\Firefox\Profiles\ and hit Enter. Find your profile folder (it's the long one with random letters, not default unless you've never customized). Inside, delete the startupCache directory. Firefox rebuilds it on next launch. This is a known fix for odd internal notices that appear after Windows updates.
Reset your preferences. Type about:config in the address bar, accept the risk, then search for browser.cache.disk.enable. If it's set to false, that's your problem. Set it back to true. Some privacy extensions flip this, and Firefox's cache layer gets confused when disk cache is off while other components still try to write to it.
Create a completely fresh profile. This is the nuclear option. In about:profiles, click "Create a New Profile". Set it as default, restart, and test. If the error is gone, it's a profile corruption issue. You can then migrate bookmarks and passwords manually—don't copy the whole profile folder, that brings the corruption back.

If the main fixes fail

Sometimes it's not your profile at all—it's a specific website's aggressive content updates. If the error only appears on one site (like a live-tile dashboard or a stock ticker that refreshes every second), that's expected behavior. The page is literally changing while Firefox is fetching it, so it aborts the first load and starts over. Nothing to fix.

If it's popping up as a notification, check your extensions again. Disable all of them, then re-enable one by one. Pay special attention to anything that touches downloads or video players. I've seen old versions of Video DownloadHelper trigger this on YouTube specifically. If you have that, update it or drop it.

One more thing: if you're running Firefox Developer Edition or Nightly, this code appears more often because those builds have more debugging output. It's cosmetic. Wait for the next update—it usually gets quieter.

Preventing it from coming back

The root cause is almost always an extension that's not keeping up with Firefox's internal changes. Set extensions to auto-update (default is fine), but also check the Add-ons Manager (about:addons) every few weeks for disabled ones that lost compatibility. Uninstall those.

Also, avoid using "Clear history on close" with the "Cache" option if you're on a slow connection. Firefox relies on cached metadata to decide whether a document changed. Turning that off forces it to re-request everything, and the resulting race condition can throw this notice. Keep the cache enabled and let Firefox manage it.

If you're a developer and you're seeing this in your own WebExtension, check your webRequest listeners. You should be returning {cancel: false} for requests you don't handle, not undefined. That undefined return is what causes Firefox to treat the request as changed and fire this notice.

Bottom line: this error is noise, not a signal of system failure. It's your browser being chatty about its internals. A few minutes of profile hygiene gets rid of it.