Is there a way to save a tab-set in Firefox?

You'd try a firefox add-on : Session Manager

If you use the Bookmarks toolbar you can put a folder there containing the links to your required sites (Right click toolbar then "New Folder" and drag required bookmarks in). Click on the folder, then "Open All in Tabs" allows you to get the required result (even with other tabs open if you don't want to close them).