So you can't easily open Firefox and investigate because it will be tied up...

Could you start by checking or changing these Windows settings:

(1) Show hidden files and folders: https://support.microsoft.com/en-us/help/14201/windows-show-hidden-files

(2) Show file extensions (e.g., .txt, .js): https://www.bleepingcomputer.com/tutorials/how-to-show-file-extensions-in-windows/

Now, we're going to check this file (don't double-click):

prefs.js

That is in your profile folder. To access that, see the part of this article about how to open the folder when you can't run Firefox: Profiles - Where Firefox stores your bookmarks, passwords and other user data.

You can open prefs.js in a text editor using either:

• right-click > Edit
• right-click > Open With > Notepad or Wordpad or your preferred text editor

In the file, look for this line. If this exists, it needs to be deleted (or the word false can be edited to true):

user_pref("browser.sessionstore.restore_on_demand", false);

If you found that line and deleted it, great, that will help. When saving, make sure to keep a plain text format.

Please repeat this process with the following optional file if you find it in your profile folder:

user.js

If you didn't find that line, hmm, why is Firefox loading all your tabs at once?!

Or, "Plan B" would be to temporarily hide all your session history files and check the browser.sessionstore.restore_on_demand preference on the about:config screen.

If you want to try that:

First, check or change these Windows settings:

(1) Show hidden files and folders: https://support.microsoft.com/en-us/help/14201/windows-show-hidden-files

(2) Show file extensions (e.g., .txt, .js): https://www.bleepingcomputer.com/tutorials/how-to-show-file-extensions-in-windows/

Next, open your profile folder. To access that, see the part of this article about how to open the folder when you can't run Firefox: Profiles - Where Firefox stores your bookmarks, passwords and other user data.

(A) In your profile folder, look for these files and:

• sessionstore.jsonlz4 - right-click > Rename to sessionstoreOLD.jsonlz4

• sessionstore.js - right-click > Rename to sessionstoreOLD.js

• sessionstore.bak - right-click > Delete (this probably is really old)

(B) right-click the sessionstore-backups folder > Rename to sessionstore-backupsOLD

When you start Firefox, it should not find any previous session history, and just show your home page.

Then:

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

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

(3) If the browser.sessionstore.restore_on_demand preference is bolded and "modified" or "user set" to false, double-click it to restore the default value of true

Then you'll need to exit Firefox and replace the useless new sessionstore.jsonlz4 file with a copy of the file you want Firefox to read at startup, giving it the sessionstore.jsonlz4 name.