Hello pamister, probably we are talking about your win xp, and not for Firefox OS (as i see in Question details), go from Tools menu > Options > General > Startup > When Firefox starts > scroll and select Show my windows and tabs from last time > click OK to save it.

see for more info : Startup, home page and download settings

also, Configuring session restore

thank you

Do you mean on your Windows XP system?

Firefox normally saves your open windows and tabs as you browse, so that if Firefox crashes, it can get you back to where you were (or close to it). When you start Firefox, you usually can return to that earlier set of windows and tabs using:

History > Restore Previous Session

You also can set Firefox to start up automatically in your previous session. This article describes where to change that setting: Startup, home page, tabs, and download settings.

However, this is not available under some circumstances:

• Private windows - not saved
• Firefox set to not remember history
• Firefox set to delete history at shutdown

If session restore is not working, you may want to check these factors, most of which are on the Privacy tab of the Options dialog. See: Settings for privacy, browsing history and do-not-track.

Can you get it to work the way you want?

Check out Send Tab URLs.