Sometimes external software will create or update an optional settings file named user.js. For some reason, one of the things that often shows up in there is a preference to restore your previous session just once. However, because Firefox uses user.js to override your normal settings every time it starts, that then turns into a permanent change.

Your More System Details (next to your post) shows that you have a user.js file. To check what's in it and/or remove it, try this support article: How to fix preferences that won't save.

Note that by default, Windows hides your Firefox profile folder from search and from easy browsing in Windows Explorer. You can access the folder from within Firefox using either

• "3-bar" menu button > "?" button > Troubleshooting Information
• (menu bar) Help > Troubleshooting Information
• type or paste about:support in the address bar and press Enter

In the first table on the page, click the "Show Folder" button. This should launch the folder in Windows Explorer.

Any luck?

Another possibility is that Firefox keeps crashing during shutdown, and so Firefox is recovering from a crash every time. To diagnose this, you can change how Firefox recovers from a crash: instead of restoring the previous session, it will show a list of what you had open so you can be selective or bypass it altogether.

Here's where you change that:

(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(2) In the search box above the list, type or paste sess and pause while the list is filtered

(3) Double-click the browser.sessionstore.max_resumed_crashes preference and change the value from 1 to 0 and OK that.

Note: Please don't change any other sessionstore preferences without researching them first.

Okay one more and I'll let you compute in peace: to run Firefox with factory default settings, try:

Create a new Firefox profile

A new profile will have your system-installed plugins (e.g., Flash) and extensions (e.g., security suite toolbars), but no themes, other extensions, or other customizations. It also should have completely fresh settings databases and a fresh cache folder. (You can import your bookmarks to it later.)

Exit Firefox and start up in the Profile Manager using Start > search box (or Run):

firefox.exe -P

Please don't delete anything here.

Any time you want to switch profiles, exit Firefox and return to this dialog.

Click the Create Profile button, assign a name like March15, and skip the option to relocate the profile folder. Then start Firefox in the new profile you created.

Does it bypass the problems you're trying to solve?

Firefox will keep using this new profile until you switch. When returning to the Profile Manager, you might be tempted to use the Delete Profile button. But... it's a bit too easy to accidentally delete your "real" profile, so I recommend resisting the temptation. If you do want to clean up later, I suggest making a backup of all your profiles first in case something were to go wrong.

Regarding migrating data to the new profile, see: Recovering important data from an old profile.

Bookmark and use this; Mozilla Search Reset {web link} This add-on is very simple: on installation, it backs up and then resets your search preferences and home page to their default values, and then uninstalls itself. This affects the search bar, URL bar searches, and the home page.

Settings Guard for Firefox {web link} Detects and resets changes to settings that are frequently done by add-ons and application installers.

Awesome. Erasing the user.js file did the trick. You are THE MAN.