Hello, click on the 3 lines or hamburger menu in the top right, then select Restore Previous Session -- it is the fourth item in the drop down menu. If this solved your problem, mark as solution.

Is "Restore Previous Session" grayed out on the History menu?

If it's grayed out, either Firefox did an automatic restoration or it was unable to restore the session history file.

If Firefox did a restoration, you can check the "Recently Closed Windows" list on the History menu for closed windows you can re-open.

Within each restored window, you can check the "Recently Closed Tabs" list on the History menu for closed tabs you can re-open.

If Firefox didn't and can't do a restore, possible reasons are:

(1) Firefox set to clear history at shutdown -- not sure this affects crash recovery, definitely affects regular shutdowns

(2) Using Private windows -- these are not stored in session history

(3) External software cleans your browser history when it detects that Firefox has shut down

(4) Firefox set not to restore after crashes (see settings at the end)

(5) Problem with Firefox's saving of session history (e.g., locked files)

For #4, could you check:

(A) 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.

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

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

(D) It's okay if the browser.sessionstore.max_resumed_crashes preference is customized to 0 -- this causes Firefox to show the "This is embarrassing" screen with a list of windows and tabs in session history instead of restoring them automatically