Was it the last window of Firefox, so when you restarted Firefox, it redisplayed that last window? You can check these possibilities:

(1) Maybe your startup setting was changed from home page to previous session windows and tabs here: Startup, home page, tabs, and download settings?

(2) Maybe some software created a user.js file which overrides your startup setting and always restores your previous session windows and tabs? This article has the steps for investigating that possibility: How to fix preferences that won't save.

(3) Maybe Firefox is crashing during shut down, so it is doing a post-crash recovery of your previous session windows and tabs? But if there are no other symptoms, then this one seems less likely.

You can also open such pages in a private browsing mode window. Then no data is stored to disk and closing all Private Browsing mode windows will automatically clear all data (cookies and memory cache).

Make sure to close all open tabs to prevent Firefox from storing session data in the sessionstore.js file. Firefox stores data about closed tabs and windows, so closing tabs may not be sufficient.