I don't think you can change the welcome page for private windows in normal browsing mode, but here are two options:

(1) Next time you have all those tabs open, create a bookmark folder of those pages by right-clicking any tab and choosing Bookmark All Tabs. Then you can use the feature to open all tabs in the folder simultaneously as the first thing you do in a private window ("Open All in Tabs" appears as the last line in the list of bookmarks in the folder).

(2) Start Firefox up automatically in private browsing mode. However, this doesn't save any history, so if you like to keep history for your non-private windows, this wouldn't be a good choice. If you want to try this, you set it in the Options dialog, Privacy tab: "Firefox will: Never remember history".

Maybe create a desktop shortcut that starts Firefox in PB mode (-private) and add the pages to its target field that you want to open in tabs if that is what you want.

• https://developer.mozilla.org/Command_Line_Options
• -private -private-window URL1 URL2 URL3

If you exceed the maximum length of the target field then use a .cmd file with the start command.

• start "" "<path to firefox.exe>\firefox.exe" -private -private-window URL1 URL2 URL3

• http://technet.microsoft.com/en-us/library/bb491005.aspx