One thing that I notice with Firefox installations in both Ubuntu and Linux Mint is that a dialogue box appears when closing down the web browser asking whether to save the open session or if you want to have a fresh session the next time that you start it up. Initially, I was always in the latter camp but there are times when I took advantage of that session saving feature for retaining any extra tabs containing websites to which I want to return or editor sessions for any blog posts that I still am writing; sometimes, composing the latter can take a while.
To see where this setting is located, you need to open a new tab and type about:config in the browser’s address bar. This leads to advanced browser settings so you need click OK in answer to a warning message before proceeding. Then, start looking for browser.showQuitWarning using the Search bar; it acts like a dynamic filter on screen entries until you get what you need. On Ubuntu and Linux Mint, the value is set to true but false is the default elsewhere; unlike Opera, Firefox generally does not save sessions by fault unless you tell it to that (at least, that has been my experience anyway). Setting true to false or vice versa will control the appearance or non-appearance of the dialogue box at browser session closure time.