There is a feature to start Firefox automatically in private browsing mode. This is a stealthy mode where the white-mask-on-purple-tab does not appear on the title bar next to the minimize, restore/maximize, and close buttons.

To see whether you have that turned on, you can check the Options page, Privacy panel:

"3-bar" menu button (or Tools menu) > Options > Privacy

If the select is set to "Firefox will: Never remember history" then automatic private browsing is turned on. To turn it off, change the selector to "Firefox will: Use custom settings for history" and then uncheck the box for Always use private browsing mode.

Alternately, the feature could be disabled or broken somehow.

In case an extension has disabled it, could you test in Firefox's Safe Mode? That's a standard diagnostic tool to deactivate extensions and some advanced features of Firefox. More info: Diagnose Firefox issues using Troubleshoot Mode.

If Firefox is not running: Hold down the Shift key when starting Firefox.

If Firefox is running: You can restart Firefox in Safe Mode using either:

• "3-bar" menu button > "?" button > Restart with Add-ons Disabled
• Help menu > Restart with Add-ons Disabled

and OK the restart.

Both scenarios: A small dialog should appear. Click "Start in Safe Mode" (not Refresh).

Any improvement?

If this problem occurred after an update, it's possible that Firefox's program files were damaged in the process, or some old files didn't get replaced with new ones. To address that:

Clean Reinstall

We use this name, but it's not about removing your settings, it's about making sure the program files are clean (no inconsistent, corrupted, or alien code files). As described below, this process does not disturb your existing settings. Do NOT uninstall Firefox, that's not needed.

(1) Download a fresh installer for Firefox 39.0 from https://www.mozilla.org/firefox/all/ to a convenient location. (Scroll down to your preferred language.)

(2) Exit out of Firefox (if applicable).

(3) Rename the program folder, either:

(64-bit Windows folder names)

C:\Program Files (x86)\Mozilla Firefox

to

C:\Program Files (x86)\OldFirefox

(32-bit Windows folder names)

C:\Program Files\Mozilla Firefox

to

C:\Program Files\OldFirefox

(4) Run the installer you downloaded in #1. It should automatically connect to your existing settings.

Any improvement?

Note: Some plugins may exist only in that OldFirefox folder. If something essential is missing, look in these folders:

• \OldFirefox\Plugins
• \OldFirefox\browser\plugins