I use histre (http://histre.com) to accomplish this functionality, not only within Firefox but across browsers, computers, and even colleagues.
My favorite feature is being able to save all tabs within a window, send a link to a co-worker, and have them open up all the tabs. It takes like 5 secs, end to end.
Best of all: if they have the histre extension as well, they can instantly open all the tabs at once and even see my highlights - it’s like being able to send a browser window state to another person!