Type about:preferences#general<enter> in the address bar.

There are two settings;

• When Firefox Starts

When the browser is started, what do you want to display. Many of us choose Show My Home Page.

• Home Page

When a new page is opened, what do you want displayed.

1. Use Current Page. Use what ever page(s) are open at that time.
2. Use Bookmark
3. Restore To Default. about:home shows a Mozilla home page with tools.

You can use any of these that you wish;

about:home (Firefox default home page), about:newtab (shows the sites most visited), about:blank (a blank page),

or you can enter any web page or about: page you want.

One strangely common reason for this is that you have acquired an optional settings file named user.js in your profile folder (personal settings folder). Firefox uses the settings in that file at startup to override your saved preferences from the previous session. Unless you created that file yourself, you may want to delete it. The following support article has a section on how to check for it (skip over the Refresh suggestion): How to fix preferences that won't save.

Note: By default, Windows hides the .js extension. To ensure that you can find any user.js file and distinguish it from other files, you may want to turn off that feature. This article has the steps: http://windows.microsoft.com/en-us/windows/show-hide-file-name-extensions

Any luck?

Thank you, jscher2000! There was a user.js there (dated the 11th, no idea what happened then that was unusual). Renamed it, and voila, back to my preferred settings.

Is there more in the user.js file apart from a line that sets browser.sessionstore.resume_session_once to true?

There were some blank lines and this: user_pref("browser.sessionstore.resume_session_once", true);

That's all.