Site not working in Firefox private mode on Linux?

A site opens normally in Firefox but shows an error, blank page, or endless loading screen in a private window. That difference usually points to separate cookies and site data, tracking protection, an extension, or a feature the site relies on.

Compare Firefox in both modes before changing settings. The Browser Info tool can show your browser version, engine, permissions, privacy details, and supported features. It cannot diagnose the site itself, but it can help you check whether Firefox is missing a feature the page needs.

Compare Firefox in normal and private mode

Open the Browser Info tool in a regular Firefox window. Note the browser version, engine, privacy details, permissions, and supported features that appear.
Open a private window with Ctrl + Shift + P. Visit the same tool and compare the information. Some privacy details may differ, but matching browser versions do not mean that cookies, extensions, or site storage work the same way.
In the private window, enter the problem website's address directly. Note whether it fails immediately, gets stuck loading, or opens but fails after you sign in or submit a form.
Open the same page in a regular window and repeat the same action. Keep the address and steps the same so you can tell whether private mode is the difference.

Private windows use a separate temporary session. They do not carry over your regular window's sign-in cookies, and private browsing removes its temporary browsing data when you close the private windows. A site that needs an existing session may therefore show a sign-in screen or behave differently without being broken.

What the result means

The site works normally, but fails in private mode

Focus on what changes in a private session: cookies, local site storage, tracking protection, or extensions allowed to run in private windows. If the page fails only after login, it may depend on a cookie or storage item that was created in your regular session. Sign in again in the private window before deciding that the site is broken.

The site fails in both modes

Private browsing is less likely to be the cause. Check the address, your connection, and whether the site is having a problem. Browser Info can also show whether Firefox supports features the site may require, but a listed feature does not guarantee that every part of the website will work.

The page opens, but a button or sign-in step fails

The main page may load while a separate service, script, or stored setting is blocked. This often happens with sign-in providers, payment steps, embedded content, and sites that store preferences in the browser. Test one change at a time so you can identify the cause.

The browser details look the same in both windows

That is useful, but it does not rule out a private-mode issue. Browser Info reports browser and device capabilities, not every site cookie, extension decision, or page error. Compare the website's behavior as well as the tool's report.

How to fix it

Open a fresh private window and sign in again. A private window does not share your regular session's sign-in cookies. If the page works after signing in, the difference was likely the session rather than Firefox's ability to load the site.
Check the site's tracking protection. In Firefox, open Settings › Privacy & Security and review Enhanced Tracking Protection. If it is set to Strict or Custom, temporarily test Standard. You can also use the shield icon beside the address bar to turn protection off for this site as a short test. Restore your previous setting if this changes nothing.
Check extensions that can run in private windows. Open Firefox's menu and choose Add-ons and themes, then review each relevant extension. An extension may be allowed to run in private windows and block scripts, cookies, or page elements. Temporarily turn off one likely extension, reload the site, and repeat as needed. Avoid disabling security extensions on sites where you enter sensitive information.
Try Firefox Troubleshoot Mode. From the Firefox menu, open Help › Troubleshoot Mode and restart Firefox in that mode. Then test the site in a private window. If it works, an extension or a changed Firefox setting is a likely cause. Return to normal mode and adjust extensions one at a time.
Update Firefox on Linux. Open Firefox's menu and choose Help › About Firefox to check for an update. If your Linux distribution manages Firefox updates, install available updates through its Software application or package manager, then restart the browser.
Clear this site's stored data in a regular window. In Firefox, open Settings › Privacy & Security, find Cookies and Site Data, and use Manage Data to remove data for the affected site. This signs you out and may reset preferences. Reopen the site in a regular window, then test it again in a new private window.
Check whether the site depends on an unavailable browser feature. Review the supported features in Browser Info and compare them with any requirements or error details shown by the site. If Firefox is current and the site still fails in both modes, contact the site's support team with the error, Firefox version, and the steps that reproduce it.

Why private mode can change a website

Firefox keeps private browsing separate from your regular session. A private window starts without your normal cookies and uses temporary site data. This protects your browsing history on the device after you close the private windows, but it can also make a site behave like a first visit each time.

Tracking protection can also prevent some third-party content from loading. A page may look complete while an embedded sign-in, map, video, or payment service is missing. If relaxing protection for the affected site fixes the problem, leave protection enabled elsewhere and use the site-specific exception only if you trust the site.

On Linux, Firefox may be installed through your distribution, as a Flatpak, or as a Snap. The browser's menus are broadly similar, but updates may come from different places. If the About Firefox screen says an update is managed by your system, use the update method provided by your Linux distribution.

A private window is not a separate browser installation, and it does not guarantee that every website will work without stored data or tracking-related content.